http://www.helminc.com or 800-782-4356 (which you could have found in the FAQ by clicking on Resources and then Service Manuals ).
You can also find a .pdf version of the 1991 service manual in the FAQ (in the Reference area) but, if you're like me, you'll find it easier to have the actual book next to you while you're working on the car.
Not sure what year you are looking for but, do not order the 1991 Service Manual from Helm, it is a photocopy in a binder. Order a 1993, it will be completely applicable and come is a perfect bound book. Additionally there is a Electrical Troubleshooting Manual and a Body Repair Manual.
I have found the Service Manual to cover 99% of what is required. Rarely do I need to spend too much time in the ETM.
You're also welcome to use my online/web based shop manual. I find it mostly useful for quick searches/part names when I'm at the office or not at my shop. At the shop, I almost always use the PDF version as the OCR program I used to convert the shop manual to text/images doesn't re-create the diagrams as clear as the paper or or PDF version.