When I bought my system a while ago, I went with the Diamond d6 700.4 amp. I am looking to add a second amp just for my subs, which will be 12 inch d6's. The diamond d6 amps my local installer has do not match. What are my options?

I don't have the money to get two new amps, so maybe I can get one online somewhere, used or new? What are good places to buy diamond amps? What models would match my amp?