Snaps are snaps, you can probably get them at a boat store, either that or McMaster.com.
That said, I think you'll find that the belt rail will interfere with the snaps, you can't put the snaps on the "lip" that sticks out, because then they'll interfere with your belt-rail top, and I doubt they'll fit in the recessed area above the lip. Place them further down the body and not only will it look bad, the tonneau or whatever won't stretch that far.
I'd just go to an upholstery shop and ask them to sew in a strip to convert your snap items to belt-rail. Or see if Chris at Badger will do it.