On ebay I just picked up all three volumes for $45 shipped, for my 98 Deville. They're out there, you've just got to keep checking. Mostly, they're higher than what I got them for, between $50 and $100. You can get them new for I think $135.
You can also try Helm Inc, they are the official supplier of these manuals, only two things to be concerned about with the paper manuals. They are no longer 'complete' GM does not include the rebuild information for the transmission, they only cover diagnostic and some basics in the 3 volume set. This other information is included in the TURM 98 manual, on HELM its about 40 dollars. The other bad thing about these manuals is they are not as up to date as they should be, there is information that is missing (ie the heated seat instructions for replacing elements) this information was published in 2003 and will not be in the manuals. I don't want to give the impression that the manuals are not worth it, they sure are but if you are paying good money they should include the updates. The other vehicles covered in these manuals are the Eldorado but not the Seville. I got mine off of HELM for 140 dollars (at the time the Canbuck was really low) and they make great weights as you will need arms of Atlas to lift them.
I have a shop manual for every car that I have owned over the past 20 years. I use the manuals about once a year--just to make sure that I am not screwing something up. No regrets, however. It helps me to learn a few things other than how to put the key in the ignition.
They do take up a lot of room; they weigh a ton. Lots of pictures too.