Personally, I think age is much more important than PR when choosing a domain to buy. If PR is a factor in your decision at least make sure it's not faked. That being said, any of the major players like Pool, NameJet, Sedo, etc... have aged domains available. I just pop the domain(s) into DnScoop or something similar to check up on their stats- I usually run them through a few tools to make sure. As an aside, I just downloaded Domain Samurai (It's free). Kinda a nifty little tool for domain research. Cheers!!