I recommend you contact some of the disease specific agencies like The ARC
http://www.thearc.org/, the United Cerebral Palsy
http://www.ucpa.org/ and the March of Dimes
http://www.modimes.org/. Even if they do not have resources they may have volunteers in your community who know of wheel chair resources.
If you have a University with a bioengineering degree program they may have a low cost to free resource for transportation.