Where to purchase older copy?

Hey all,

So I'm interested in purchasing after effects both to improve the short films I make as well as to give me more creative outlets, but my budget doesn't allow for production premium 6, so I'm hoping to find a cheaper copy of 5 or 5.5 online somewhere. There are a few sites which claim to have it but I'm a bit hesitant to fork over hundreds to a site I don't recognize. Hoping someone out there has some advice for good places to look to find a legitimate copy. Thanks!