Depending how much you need, you might also try http://www.gotgrit.com/ -- they sell primarily to telescope makers for optical grinding, so the quantities might be a bit large, but they're certain to have what you want. If you're making a ground glass, you could probably finish the whole job with a 35 mm film can full (I used less than that in this size for an 8" telescope mirror, though the surface was ground to 320 before I started with the 500). The lapidary shops will almost certainly sell in a minimum of a half pound, too, so take your pick...