From time to time you can get them on eBay, though most are probably in the hands of collectors by now. LOAF and Joe of the Origin Museum got their stuff mainly through contacts they have at Origin. At the first Dragoncon, for example, Mark Day (producer from WC4 and WCP, I believe) offered to clean out his garage for props and send them to Joe. Myself being local, he shipped them to me so I could deliver them at the Con. It was like opening the best christmas present ever. In fact, I still have the shipping invoice on display with my WCP helmet.
You can try contacting folks like that directly and asking if they know of anyone hoping to unload such things, but chances of finding something are probably pretty slim about now. It's all about who you know.