MacBookPro8,2 (early 2011) RAM size limit per Bank
Apple states that you should fill both RAM banks with equal size memory for best performance, implying that one bank should work.  Your machine running on one bank would seem to bear this out.

For the second part of your question I can recommend RamJet (1-800-831-4569) as both a good source of quality technical support that is Mac specific, and a retailer of RAM.

I have no business connection to them, but I have been using them for my own machines and my clients machines for almost ten years.