You should already have one. Look at the lower grille opening (where the three small openings are). Directly behind it, you should see a small radiator with an input and output hose on the driver's side. The height of that radiator should be roughly the same as the lower grille openings.
The "axillary" transmission cooler you are referring to only comes with the V6 and Trailer Towing Package.
Shop online (eBay), sometimes you can find a genuine Ford cooler for not much money. They are easy to install. Any good auto parts store should ATF hose, but you can probably use high pressure fuel hose in a pinch.