Nomadcat and Glider, a simple way to get a tranny temp gauge is just to run a Scangauge II via your OBDII port, which will give you lots of other good info anyway. Some have argued they aren't completely accurate on tranny temp but to me exact temp doesn't matter. What you want is to be able to monitor when it's rising and/or high and react accordingly. After you drive with one awhile you won't leave home without it and you'll know the norms of your transmission.
If you don't tow a trailer I'd skip the aftermarket cooler until you run one of these awhile and gauge temps. Chances are that an extra cooler is overkill for a properly cared for tranny.