tranny cooler

I hear all the talk of a tranny cooler, where and what's the best price on one and is it easy to install myself? Also, what's the thought of when to change and flush the tranny oil (every ##,### miles?) I will also start using synthetic oil. I had a 90000 mile check done last October so I will have to go back to my records and see what fluid they used.

afsquid
green '92
104K