Check with your travel agent before leaving. He knows the best places to go, how to get discounts and which ones are the cheapest, yet still offer quality products and services. You travel agent will be able to help you find the different spas in the area and get a menu of services offered and the prices. You will then be able to compare the different spas and find which one offers the services you desire and which one is the cheapest.
Ask the front desk or concierge at your hotel. Employees of the hotel know where to send guests and also have coupons and special rates just for guests at the hotel. In the end, you may not be visiting the cheapest spa in the area, but your price will be cheaper because of the discounts you may receive from your hotel's concierge. Employees at the hotel also have insider knowledge of the area because they reside there and may be able to point you in the right direction based on personal know-how.
Look online for some area spas, services and discounted rates. Sites such as citysearch.com, smartertravel.com and expedia.com offer a listing of spas, hotel spas and other spa-type businesses based on location and price. Citysearch.com offers a comparison and review of spas in Philadelphia.
Call the Philadelphia Chamber of Commerce or the Philadelphia Convention and Visitors Bureau. Both of these locations have listings of spas in the area and may also offer discounts and special rates for visitors. Sometimes, the Chamber of Commerce will also have coupons to offer visitors and newcomers to the area to help boost business of a particular spa that is a member of the Chamber.