400 should be fine.
woodcrafters smallest grit is 320. sounds like a neat store though. might get a pack of the 320 to try and have a look at that store.
O'Reilly has mostly stick on's rather than hook and loop. My orbital sander is older. maybe stick ons have replaced hook and loop?
I can always order some 400 from Peachtree. No biggie...
Karin