Exia gave you all the right places to search that's for sure.
if you can afford a new upgraded trans then by far, that's the way to go.
if you know how, you can rebuild it to be bulletproof yourself
If your truck has the 4R100 transmission, then I suggest a rebuild, these transmissions are rated for 10,000 lb towing cap. and are the common transmissions used in their larger fleet vehicles. they can also be upgraded with shift kits(ranging from mild=$40 to extreme=$150) not to mention all the internals have aftermarket counterparts
we are really lucky with these trucks in that we have such a wide choice, find out what you need your trans to do for you, (towing, hauling xss, etc) then see if anything in the stock realm meets your requirements, if not then it just comes back to comparison price between new (like from summit, or mod max) or rebuilt to be bulletproof.
It sounds like you have made up your mind already on buying a pre-built pre-modified trans for your rig. I think that's cool as hell.
if you tell us what kind of trans you have, it will help us narrow down your possible upgrade options, if your truck hasn't changed hands a lot before you got it, you can probably use and trust the door sticker to get the trans code. It will likely be underneath the letters TR. use this if you need a visual helper
1. Exterior Paint Code
2. Region Code (not listed)
3. Domestic Special Order Code (not listed)
4. Wheelbase Code
5. Brake Code
6. Interior Trim Code
7. Tape/Paint Pinstripe Code
8. Radio Code
9. Axle Code
10. Transmission Code
11. Spring Code
(credit to SHOTGUN at f-150forum.com for the above picture and key)
you can also use your VIN to acquire the same information.