As someone who is both an artist and an art teacher, I have long relied on this company and highly recommend them to my students as a source of mail order art supplies:
UTRECHT ART SUPPLIES
They manufacture their own paints as well as sell popular brands made by others. Their paints are both reasonably priced and high quality. They do market both student grade and professional grade so prices will reflect that.
They offer a variety of canvas - I highly recommend the 100 percent polyester, which is a bit more expensive than cotton canvas but much easier to deal with if you're planning to stretch your own. Utrecht used to be the leading importer of fine Belgian linen and other types of canvas and they still offer a full line.