Take a look at the Comprehensive US OP list, which cross-indexes most of the OP's discussed on DB, along with the pharmacies that actually fill the scripts You can find it on the drop-down navigation list at the top of the page. It can get out of date pretty fast, so be sure to check the threads on the OP's you are thinking of using, that's where the most up to date info is.

Lots of OP's seem to have been changing Pharmacies lately, starting with the Pharmanet meltdown that sent several of them scrambling to fill scripts.

And like Tansun says, do your homework! It's amazing the info you find when you read the threads.