OK, after doing some research, here are my favorites online ~ I've ordered from two of them, but I can't remember which two!
Mosaic Tile Supply - near my part of SoCal
They have TONS of neat items, not only the basic 3/4" tiles: tiny 3/8" tiles, absolutely stuning Monet Glass Shimmer tiles, precious metal tiles, China sets (broken plate sets), soft edge stained glass pieces, porcelian flowers, polished abalone shells, beach glass, sea shells, and bulk mixes of both random tiles and glass. Decent prices.
Stuff to mosaic ON - plaques, mirrors, frames, clocks, vases, candle holders etc. Also sells mosaic supplies like colored grouts (which I personally have a difficult time finding locally for some reason), adhesives etc. Also has tiles in the shapes of hearts, rounds, gems, funky tile mix (you gotta see it!), really neat clear glass squiggles, glass stars etc. I think they have decent prices.
Lots of tiles and stuff, but expensive. You can also find their stuff in craft stores like Michaels in little tiny packages that cost big bucks
Mosaic Tile Artswww.mosaictilearts.com
I think they have good prices - between $2 per lb/200 tiles and $3 per lb/300 tiles; or 2 cents each. The tumbled glass is a good deal at 1 lb for $2.90 or 5 lbs at $12.50. It's an easily navigatable site, where the tiles are organized according to color family ~ I like that
And at www.mosaic-witsend.com
you can get nice glazed freezeproof & waterproof outdoor 1"x1" thick tiles. 1 lb/57-tiles of a single color is $10. Kinda expensive, but if you're doing outdoor work large-scale and want some neat pieces, this would be the place to shop at.
Have fun tiling!