Plastic mulch can be hard to find; the black cloth seems to be more widely carried, and "breathes" better. You might try a Farm & Fleet (there are several regional variations) or farm coop, if you have one near you.

The best mail order sources I have found are Peaceful Valley Farm & Garden Supply (California)(link), and Johnny's Selected seeds (Maine). They both sell in bulk, so you will still have to shell out some change; but you won't have to buy it again for at least a few years (and that is if you garden heavily).

Here is a link that might be useful: Peaceful Valley