It depends on how many of a certain size you want at a time. I get some of mine from Griffin Greenhouse supply here in Knoxville. I have found them willing to match prices if I find them cheaper elsewhere (online), but their prices are usually pretty well in line with other sources. I only buy in whole-case quantities from them.

If I need only a few of a specific size, I sometimes am able to find them at Knoxville Seed. I pay quite a bit more there and the selection is not great, but sometimes it's a good option.