My first love is mosaics, but I also do a little furniture repurposing.
For mosaics, the site I use most is
http://www.mosaicbeach.com (in Toronto). I mostly buy vitreous glass there. But by far my best shopping is done during the summer at yard sales. I am always looking for dishes to break (way therapeutic) and cut into tiles, and also furniture, mirrors, shelves, picture frames . . . anything that I can mosaic.
I've also gotten tons of great stuff for free by running ads on craigslist asking for broken dishes, ceramics, porcelain . . . and have snagged mosaic supplies this way too, from people giving stuff away.
Dollar stores are great for glass gems, small paint brushes (for applying glue, not so much for painting) and all kinds of other treasures.
Probably the greatest resource for things like (Oops) paint, drawer pulls, tiles, grout . . . is the Habitat for Humanity Restore. If you have one in your area or anywhere in driving distance, it's well worth the trip. Everything sells for a fraction of the price you would pay elsewhere, and (at least in Montreal) there is no tax. It would be the first place I'd go if I was doing any kind of home reno project because they have everything from sinks to doors to tools . . . anything that goes into the building of a house is likely to be found there for peanuts.
Home Depot is a fabulous resource for Oops paint (paint that doesn't turn out to be the colour the customer wanted and is put up for sale) especially if you like bright colours. I can get a gallon can here for $9, compared to the $40 something it goes for usually . . . you just have to check it regularly because it's hit or miss. (Actually most paint stores sell some Oops paint as well, just not in the same quantity as Home Depot.)