Craigslist... does Puerto Rico even have Craigslist?
Go to various horse selling websites, put in "Puerto Rico" and see what happens?
Contact the AQHA and ApHC and ask if they know of any breeders in Puerto Rico?
Curious as to why you want Quarters and Appies specifically though, both are wonderful breeds... but they're dreadfully common on the US mainland. Why not spice things up and see what the local breeds of horse have to offer? You're in the homeland of the Paso Fino! Why not give those pretties a try?