Have you checked with your local Council? I have also asked @ shopping ctrs, car dealers, tyre repair places (they often have accessable ppoints outside), pretty much anywhere I ride. I just tell them it will cost maybe 30-40 cents for a couple hours. Most say OK.
I also usually leave some brochures on the seat of my scooter for any passers-by. I feel it's just great to get the info out there.
And what a great door prize!