Mosaic Craft Supplies
This site offers a nice variety of mosaic bases, many aimed at kids and beginners.
Mosaic Art Supply
Among this site's mosaic making supplies is an amazing collection of tiles, stones, glass, marbles, and other tesserae.
This site offers mosaic tile and mosaic supplies specially selected for crafters, artists, interior designers and teachers.
You can buy any mosaic supplies you need on this site, including kits.
Their mosaic supplies include a collection of fun wooden shapes you can decorate.
Stone By Stone
This site offers several ready to use mosaic supplies. Great for kids.
Wits End Mosaic
Whether you're a hobbyist or homeowner, artist or designer it's their goal to provide you with materials that inspire you.