You can ask your friendly neighborhood tax examiner to take a look at their commercial activity of renting out parking slots without the issuance of corresponding receipts. AFAIK, that's a violation of our tax laws. Hindi nag-dedeclare/report ng tamang income yan! Also, go to the Business Permits and Licensing Office of your city or municipality and check if they have a business permit for leasing out parking slots. Malamang wala rin. This'll give them more headaches faster than you can say "boo yah!"
Seriously, just find a unit owner who's not using his parking slot and rent it out from him/her. Chances are you'll get it at a lesser cost.