Craigslist, E-bay, Amazon comes to mind. You'll just have to look alot. I don't know if there's alot of used plates out there or not. Maybe check salvage yards? I'd think alot of people would leave the base plate on and not bother to take it off when getting rid of their Libertys. So there may not be many used ones for sale.
Steve & Nancy
2005 Itasca Sunrise 33', W20 Chassis, Ultrapower, Henderson Trac Bar
2012 Chevy Captiva Sport AWD, ReadyBrute Elite Tow Bar, Blue Ox Base Plate, Protect-A-Tow