Hi Dan,
I've owned CVRTs for 10 years nearly now. You will have no trouble finding bolts, bearings etc that fit but I don't think these are the parts you'll be changing too often. The bits you will wear out are wheels, sprockets, track, belts, brake pads, master cylinder kits and filters. While it's possible to find some standard off the shelf replacements for some of these items the rest are vehicle specific and usually need to be bought from military surplus dealers.
CVRTs are great fun, the Spartan is also the best variant for taking people for rides etc, so don't let this put you off . There are other owners in your area shipping things from the UK regularly and my advice is to share some container space and buy yourself a kit of basic spares to keep on the shelf. Some spare track, a set of wheels etc is much cheaper to ship this way and while fairly easy to find right now in the uk may get harder once the vehicles go out of service.
Chris