I found mine on Craigslist and it was also posted in the Classifieds here on Daysailer.org. www.sailingtexas.com has listings from all over the country and often has a DS or two.
Local sailing clubs and the local DS Fleet are other places to look - sometimes at our club a boat will go up for sale on the clubhouse bulletin board that is not listed anywhere else. I don't know if any of them make it their, but you might also check boat specific sites like yachtworld.com, sailboatlistings.com and boattrader.com. They seem to be dominated by the bigger vessels, but there could be a dinghy or two hiding in there somewhere.
Most of all, just keep looking - took me a while to find mine, but with 10,000+ out there they come up pretty often. Good Luck!