The H Plus Son rims have a good following. I don't think I've ever heard anyone say anything negative about them.

I've used Sun CR18, M13-II, and M14A rims. Other than one particular rim with an unevenly joined joint, I've been completely happy with them -- especially for the price. The polished silver ones look nicer than a cheap rim should.

Another cheap but decent rim I've used is the Weinmann LP18. My only complaint is that I don't care for the anodized finish.

I've bought rims from a few online sources, depending on who had good prices on what I was looking for: I'm pretty sure I've ordered rims from Niagara Cycle and Bikewagon. Maybe Modern Bike, too. Universal Cycles seems to have a good selection and a good reputation.