Here are a few of the sites I was looking into before we were able to buy my travel wheelchair:
https://www.scootaround.com/cruise-rentals/cruise-ports (shows San Juan as a port they service)
http://www.specialneedsatsea.com/reserve-equipment/
When I was looking, I was thinking about getting one delivered to our hotel in Galveston (they also deliver to the boat, and it would be sent to your cabin) and then picked up from our hotel in San Juan. Just a thought.
Let me know if you need any other information about getting around (taxi's, transfers, handicap accessibility, etc) with the wheelchair.
~Tink~