Three good options to consider:
Ultra CS3 for good keys in a separate program, easy and fast.
Keylight in AE, very good keyer, pretty steep learning curve.
Primatte is often mentioned as well as a third option.
Downloading you have to do yourself.
Unless you have the CS3 suite, it'll be hard to find Ultra CS3. Keylight is included in After Effects and is excellent. Primatte doesn't really offer anything that Keylight doesn't have. Keylight is your best bet.
BTW - Premiere Pro has a 'chroma' key among others (several in the keying family) but I'd be lying if I thought any of them were actually really good.
However, they're already there so in the effort of completeness, I mention them as well.