If you want to buy a factory service manual, http://www.helminc is the place to go. That's where I bought mine.
There are also CD and DVD manuals on eBay from time to time, and for a lot less money than the printed manual. But some require you to purchase a license to view the information (which they don't tell you in the eBay description).
www.autozone.com also has a pretty nice step-by-step online repair guide for most cars. In a lot of cases, it's easier to follow than the Service Manual for my '95.