i need a motor for a 1987 ford f 150 motor size 351 or 302
Try posting on craigslist under wanted, or check out ebay. If you don’t see an exact model, sometimes you can message the seller and they might happen to have the one you’re looking for.
If you know the specific car models that have it, you can call your local car junkyard to see if they have any of those cars in stock — usually cheaper, too (though you might have to rebuild a bit).