Have you inquired via google? I would suggest the hotel or resort where you are staying, too. They should have info for you. I have never been there, but the Cabo Wabo bar, owned by one of the Van Halens (or maybe Sammy Hagar) would be a place to hang out and inquire about bike rentals among the patrons or employees.
I couldn't imagine having too much difficulty finding a rental. Keep us posted on what you find!