I've got a 3-point disk, but... it's not for sale... besides it being in Texas.
Around here... you can buy a new one (for about $500) at Tractor Supply. I don't know if you have any such stores near you (Tractor Supply, Rural King, Farm and Fleet, etc.). I'd also recommend checking your local craigslist... and perhaps the classifieds on other farm-related sites (i.e. www.ytmag.com). If you're unable to find a used one... try one of the retailers that I listed above. Northern Tool also sells tractors and implements... and will probably have one.