I don't think they ship to the US do they? Not directly, anyway. You'd need to check that.
Things i've done or added so far have been: Change detonator plastic clip to aluminium clips, change belt for canvas belt, bought boots (and painted), made neckseal, made leather holster, bought blaster.
Apart from a few minor tweaks, I don't think there's much else to do for basic 501st level.
Have a look at the EIB requests page - There have been a couple of Spanish guys who have submitted requests with SDS armour - Maybe those threads will help.