Bachmann makes G stuff in their regular line and Spectrum line that should fit. One problem with G is it’s really narrow-gauge - 1:24 scale trains running on No.1 (1:32) gauge track. For what you want, you’d probably want standard gauge so might be limited. Some companies make 1:29 scale models that are kinda half-way in between. Otherwise in O scale you’d probably find a lot more stuff, craftsman’s kits etc that should fit the Civil War era.
<>BTW you’d probably find more responses if you post this over on the Model Railroader or Garden Railways forums. Classic Trains is really for discussions on prototype (real) vintage trains. [:)]
MR did a real good story on USMRR cars and engines…leaning heavy toward the B&O of the period…early 70’s i think…but uh…have to gemme a day or two to find it scan it etc etc…but ill get it