Since you're having issues finding fans in the first place, you can't really choose anyhow. I'd just grab whatever fan you can (from the known ones that work ok or from a V8) and try it out. Cougars, T-birds, and Volvos are other common donors.
For searching, you have to be specific. For instance, no one on this forum would refer to a fan as 'making the most CFM.' CFM is a rate of flow, and people don't say what flows the most because the best ones are seldom used or known (i.e., the Viper fan is one of the best out there and you seldom hear them mentioned, though they are amongst the best).
A simple internet search for "Electric fan for Mustang" brings up lots of info. If a link or article you read looks like it has a fan you like, search again for specifics for just that fan. Searching like this tends to snowball as you keep finding links inside links and you end up finding the info you want.
Good luck.