Generally speaking the best prices are from OEMs who specialize in hardware but stock some of the popular software titles. They tend to sell off old titles very cheap.
AutoCAD 2000 might be to old for them to still have laying around (it's practically abandonware)so your best bet may be eBay or wait another month or two for AutoCAD 2006 to drop in price (but double check the TimberCAD board here and make sure there aren't issues with installation).
If you have computer shows come through your area that's another good place to pick up old software titles.
