Try the following links:
www.screamandfly.com (check the forums for boats over 21')
www.offshoreonly.com (check classifieds)
www.boattrader.com (couple of 21's in there now)
Also, if you are interested. I can send you the info of someone who might be interested in selling his. I test his up in Seattle earlier this year. It was a white 90 with a 97 2.5 O/S. It was pretty clean and pretty fast..I just wanted something newer and they don't make the 21' anymore.
Another option is to look at the 21' Lightning; they are made in Sarasota, Fl and are a splash of the Skater 21. I test drove one of them also, and they have made improvements on the bottom with an additional lifting strake, so the boat has more lift than the original skater and does not hunt at lower speeds. If I was getting a 21' I would definitely look at the Lightning mini cat 21'. It will handle the rough better than a DCB Mach 22, Cougar 22, or Eliminator 22 hands down, and will drive better than the old Skater 21. The Lightning, nor the Skater were rigged to the level of a DCB, but both are fast and fun rides...You can find used Skaters for anywhere between 15K-30K...A brand new Lightning will go for 39K-42K with a 2.5 or Promax 300. Talk to John at Lightning; he was a pretty good guy...just had my heart set on a bigger cat.