Stephen, you've hit upon one of the prime logistical problems in teaching scuba: how/where to find a pool!
There have already been some good answers... local colleges, high schools. However, you should also try municipal aquatic centres or even private or athletic clubs.
A couple of points:
1) Renting the WHOLE pool is VERY expensive. Sometimes all you really need is a lane with a deep & a shallow end. If all you want is "a bit" of the pool your scheduling options will be wider, as some pools will let you "share" with classes that they are already running.
2) Offering a discount on scuba instruction to the facilities. If you offer to teach staff for a discount, they usually will reciprocate with good pricing & availability.
3) If you go the private or athletic club route, access is easier if you offer to teach their members (or staff) at a special price.
How to overcome objections: offering a "damage deposit" is a good idea. You can go one better by making sure that they know you will be using groundcloths, tank boots, etc. to protect the pool surface.
In the end it's a positive attitude and willingness to co-operate with these places that forges the relationship necessary to run a successful pool program with an outside facility.
Hope this helped,
~SubMariner~