Since you have the top of Town Hall, I would first try to expand it to a charity haunt. At the end, a percentage of the funds will go to a select charity.
Second, I would hit your local halloween stores(Halloween USA, Halloween Express, ect) and see if they would be interested in a sponsorship of your haunt. If it is for charity or for a cause other than self profit, they would be alot more willing in helping out.
As for the walls, black plastic is the easiest and cheapest way to go. Walmart has rolls of 10'x25' for $7! If you look in their lighting department, they also would have black lights dirt cheap. The same with strobes.
www.qvc.com has complete fog machine kits for $30.
www.ebay.com is a very good source for some props. You can get many things for under $5, plus shipping.
www.buycostumes.com has good deals on masks and spiritgum.
www.frightcatalog.com also does alot of specials.
Finally, www.hauntproject.com is great for how-to's.