Everything Fredrick said is true. I relic hunted in Petersburg in the 1970s and liked to hunt CS trenches. The most common buckle I found was the standard US. I have seen one rectangular CSA buckle that was found near Five Forks. Not a lot of brass CS buckles of any type seem to come from the trenches when compared to US and iron roller buckles. The Confederates were well supplied by Grants Army as a result of the Overland Campaign. Since you are getting your reenacting impression up these pages may be of some help.

http://www.angelfire.com/ma4/j_mayo/csequip.html

http://www.angelfire.com/ma4/j_mayo/csequip/csbelts.html