I use the "Supplement to the 'O.R.'" all the time in my research and writing. The set is divided into two basic parts: reports that were omitted from the original "O.R." back in the 1880s and "Records of Events" which is regimental and company itineraries for most of the Union and Confederate units. Fortunately for me, a nearby library has the full set. I encourage you to call some of the libraries within reach, especially the colleges, and try to get a look at this set before you cough up the money. While you are there, jot down the volumes that hold the most interest for you.
Contact Broadfoot Publishing Company in Wilmington, NC via their website and see what they have to say about their landmark book set. Who knows, but perhaps they may sell you just the volumes that hold the most interest for you (such as the ones containing all the itineraries of the Missouri regiments of both sides). Don't tell them that I said that. Besides, Broadfoot is one of the finest Civil War publishing houses for reference works, and you may find more of interest there.
If you still wish to purchase, you may try Abebooks online used bookstore and others.