Most people don't realize that postage stamps can be purchased for less than the price at the post office. The key is stamp collectors, who often buy in bulk in order to find and save particular stamps. At our stamp club in Madison, WI, anyone can buy stamps at about 85% of their original cost.
There is no central database for locating clubs of stamp collectors. The two easiest ways are to call a local stamp dealer to ask when/where the stamp club meets [he/she may offer to sell you the stamps at a discount himself], or, more likely, call or visit your local library, which should have a list of local nonprofit groups and community activities. Most stamp clubs meet once or twice a month in a public place, and if someone has a surplus of postage they will typically be glad to unload it.
Another way to do this (especially for larger lots of stamps) is to buy them on eBay. Search in the category "stamps" for "discount postage".
The only downside is that you may only be able to get older stamps, so instead of affixing a 41c stamp to an envelope, you may need to use a 15c and two 13c. That means some extra licking - but if you're going to need $200 worth of postage and can buy it for $175, why not do so and spend the difference for pizza for the volunteers?