All At Sea is available online, as is Caribbean Compass
. Both have classifdes in the back listing used boats throughout the Caribbean
... inclusing Puerto Rico
But if I were you - I'd just go over to Fajardo and walk the docks at Marina del Rey. Speak to the yard manager and let him know what you're looking for.
Or take the seaplane to St Thomas and speak to Bruce or Terry or Maritime Yacht Sales at Independent Boat
Yard... or Jose at St Thomas Yacht Sales at Compas Point. And tell 'em I sent you.
There are LOTS of boats that fit your criteria in both PR & the USVI.