Mosaic tesserae
Maryland Mosaics - vitreous glass, recycled glass, beach glass and more, Weldbond and Mac glue, styrofoam forms, lots of tools, and one of the friendliest and most helpful owners on the planet.
Wit's End Mosaic - more awesome glass supplies, Cinca unglazed porcelain, great selection of tools and pre-colored grout (much richer colors than adding your own pigments).
Dynamosaic - great 100% recycled glass tiles in small format and amazingly rich colors
Di Mosaico - Italian smalti heaven!!! 291 colors, not including the gold colors.
MosaicSmalti.com - Orsoni supplier (smalti), and another one of the most wonderful people in the business. She's also an outstanding artist - check out Lynn Moor's artist website
Tessera Glass - all kinds of goodies here, but I especially recommend the Venetian glass tiles: rich color, very few bubbles, and cut like butter.
Tierra y Fuego - *drool alert* absolutely delicious Talavera tiles
Delphi - Stained glass from many different manufacturers - shipping costs are painful, but they are meticulous about getting it to you safely. Never had a single crack or chipped corner.
Fire Mountain Gems - wonderful source for semi-precious gemstone beads and glass beads
Painting and Framing Supplies
Dick Blick - you name it, they have it. compare prices, sometimes they're crazy good, sometimes........meh.
Jerry's Artarama - my second favorite supplier - sometimes they don't have everything I'm looking for.
American Frame - great source for reasonable canvas floater frames in metal or wood.