Bryce,
Your best bet for individual units is probably Wallace: A Guide to Virginia Military Organizations 1861-1865
As to the organizations of the reserves in general, here is what I've found:
https://ia801406.us.archive.org/26/items/sketchesofactsjo00virg/sketchesofactsjo00virg.pdf
[Militia being replaced by Reserves] This act was passed Feb 20, 1864 according to-Wallace: A Guide to Virginia Military Organizations 1861-1865
Sketches of the Acts & Joint Resolutions of the General Assembly of Virginia 1863-64
10. An act vacating the commissions of militia officers
Vacates the Commissions of all militia officers, except o£ officers of the second-class militia and of the 19th regiment of the militia of the line.
11. An act disbanding the 179th regiment of the militia of the line, and for the more efficient organization of the 19th regiment of the militia of the line and the 1st regiment of the second-class militia.
Authorizes the appointment of a board of examiners by the governor, to pass upon the competency of officers of the militia. Provides for the arrest and trial of officers or privates for neglect of duty. Disbands the 179th regiment. Authorizes a new regiment to be formed upon certain conditions.
The war of the rebellion: a compilation of the official records of the Union and Confederate armies. ; Series 4 - Volume 3 pp 178-181
March 1, 1864
The war of the rebellion: a compilation of the official records of the Union and Confederate armies. ; Series I, Vol 36 part 2 page 1012
SPECIAL ORDERS, No. 113. ADJT. AND INSP. GENERALS OFFICE, Richmond, May 16, 1864. I. Brig. Gen. James L. Kemper will take immediate command of the reserve forces of the State of Virginia. He will complete their organization and place them at once in service. To this end he is authorized to employ all enrolling officers, who are hereby directed to obey his instructions. He will establish his headquarters at such point as he may deem best. Officers of the Quartermasters, Commissary, Ordnance, and Medical Departments are required to furnish all necessary facilities. All officers from the State of Virginia of the Invalid Corps and such of the regular forces as are for any reason unassigned will immediately report to General Kemper, who is authorized to assign them temporarily to duty with the reserves
The war of the rebellion: a compilation of the official records of the Union and Confederate armies. ; Series 4 – Volume 3, pp 558-559
HEADQUARTERS RESERVE FORCES OF VIRGINIA, July 26, 1864. General S. COOPER. Adjutant and Inspector General: GENERAL: I respectfully submit that the generals of reserves for the several States should be authorized to effect some organization of detailed men for military purposes. It is already settled that these detailed men constitute part of the reserve forces. Eight thousand men have already been detailed in Virginia; besides these there are at least 25,000 applications for detail pending in this State. There are 11,000 cases pending in two Congressional districts alone. Most of these are able-bodied men, and could render good service in emergencies. Now, it will not do, as a general rule, to incorporate these detailed men with the permanent companies of reserves. A reserve company is but seldom stationed for service in the county in which it was raised. Some of them are now serving at a distance of from 100 to 200 miles from their homes. A raid may be made upon a county when the reserve companies of that county are thus absent upon distant service, and there being no officers at home to call them out and embody them, no organization to which they could attach themselves, they could not be made available for local defense. It therefore becomes necessary to organize these detailed men into local companies at home. In this way only can they be armed, drilled, and made fit for service. Besides, if these local companies are formed many able-bodied men who are exempt from all military duty will join them, provided the guarantee is given that the companies Will not be called out except in emergencies and for the defense of their own section. I have ascertained that in this way I can raise 300 additional men in Halifax County alone, who could be quickly moved to Staunton River bridge in case of another attack on that point. In obedience to your suggestion I enclose the rough draft of an order which would accomplish the objects indicated. I have the honor to be, very respectfully, your obedient servant, J. L. KEMPER, Brigadier- General
[Second indorsement.] JULY 27, 1864. ADJUTANT-GENERAL: The idea is approved. Let the order be thus: All detailed men, including those between eighteen and forty-five years of age, being in the military service, should be arranged into companies and appropriate organizations by the commander of the reserve forces, to whom they will report, and who will command them. They are not expected to be called out except in emergency, occurring in or near the counties of their abode; nor will service be exacted of them beyond that county and those contiguous thereto. All exempts will be allowed and are invited to enroll themselves with such companies, so as to be prepared to aid in defending their homes when menaced by the enemy. J. A. S.
[Enclosure.] All detailed men, including those between eighteen and forty-five years of age, constitute a portion of the reserve forces. Their names will be borne upon the rolls of the companies of reserves organized in the company districts in which they severally reside, and will be called out for active service with their respective companies in and during emergencies; or the said detailed men may, at the discretion of the generals of reserves, be organized into independent companies for local defense and special service. The local companies of detailed men which may be organized under this order will not be required to perform service beyond the limits of the section in which they were severally raised, and will not be called into the field except in and during emergencies for the defense of their respective sections. They will be armed, disciplined, and regularly drilled under the direction of the generals of reserves. All persons capable of bearing arms who are exempt from military service will be allowed to be mustered into the local companies authorized by this order.
The war of the rebellion: a compilation of the official records of the Union and Confederate armies. ; Series 4 – Volume 3, pp 654-655
HEADQUARTERS RESERVE FORCES OF VIRGINIA, Richmond, September 16, 1864. General S. COOPER Adjutant and Inspector General: GENERAL: I have the honor to forward herewith the muster rolls of the reserve forces of Virginia which have been organized up to this time.
The muster rolls contain the names of 13,072 officers and men. They show that the reserve forces of Virginia, already organized, are composed of 144 companies, averaging 90 men to the company. They are already divided into five brigades under temporary brigade commanders, embracing nineteen regiments and battalions, besides detached companies. These brigades have an average of 2,600 men each. I have the honor to be, general, very respectfully, your obedient servant, J. L. KEMPER, Brigadier- General.