You can source shells from drummaker.com , but there really aren't a whole lot of off-the-shelf options for marching snare parts. You're going to need rims, tension rings, posts or lugs, snare strainers, etc. Unless you've got the machining facilities to fabricate them, you'll need to buy parts for
Dynasty/
Pearl/
Yamaha/etc. drums, and that's going to ultimately cost you more than simply buying their drums.
I think it would actually be easier (or at least less expensive) to start with basses and tenors since you can get all the parts off-the-shelf.