You'll need to find the right one for your truck's year, engine, and options. eBay would be a good spot (several reputable sellers), but it might take awhile to come across the right one.
Just search for 'ford f150 91 computer' and you'll find a few. 1991 is a transition year, so it might be a little harder to find one than some other years.
You can get them at your local parts store too: