Another good guide is the Metropolitan Opera's Guide to Recorded Opera. Unfortunately, it is out of print, but you may be able to pick up a copy on eBay or Half.com. The guide only covers operas recorded prior to the early 90s, but covers most of the classic recordings. While everyone one has there own personal taste, if you get one of operas the book recommends, I doubt you'll be disappointed.
As a final note, if you can't find a specific opera you're looking for, try Amazon UK or Tower UK. I have picked up some great recordings that are no longer available in the US.
As a final note, if you can't find a specific opera you're looking for, try Amazon UK or Tower UK. I have picked up some great recordings that are no longer available in the US.