In the 1990 issue (October) of the Confederate Calendar, Capt William Trower of the 11th Missouri is pictured with his niece. Capt. Trower is wearing a shell jacket with brass "Block I" buttons and a "CS rope-border" waist belt plate. Because the 10th and 11th Missouri were brigaded together from Prairie Grove to their surrender, this is probably a good indication of how many of the rank and file were dressed.
Having found several of Maj. Gen. Mosby M. Parsons' 1863-1865 camps in Arkansas, I can tell you that iron-back "block I" buttons are often found. Isaac & Campbell knapsack parts have been found in the camps. Acouterment plates that have been found in Parsons camps include iron "CS ovals", brass "CS eggs", brass "CS rope borders", brass "Leech and Rigdon two-piece CS" plates.
Whether they were this well dressed at the time of Prairie Grove is anybody's guess; however, by the time of the Battle of Helena they were likely equipped as such.
I hope that helps.