Davey's Jeep Parts comes to mind too but for the wrong reason.... WAY overpriced Jeep parts. I priced a used Rubicon locker just last week and Davey's wanted $750 and wouldn't negotiate!!! That's as much as a brand-new ARB locker costs.
One good source is Craigs List. And if you don't find what you want in your local area, you can search for third-party Search engines that crawl various Craigs List locations outside of your local area.