Man a tight budget, no local go kart shop and no mail ordering is making it kinda tough to help you out here. Maybe you should look around for a used go kart for sale that already has the axle, wheels etcetera. I've seen several complete karts (w/o engine) selling around here for under $50 lately. Your probably going to have to pay more than that for just a couple decent rear tires.
If you have a metal supply yard they may have some hardened 1" diameter round stock available, you could then have a machine shop broach a 1/4" keyway into it so you could mount regular go kart style components. But then again unless you have a local go kart shop you'd still probably need to order the rest of the axle components from somewhere...