Other than ebay of course, I found a website www.actiontoys.com
and they have a lot of stuff like that. I browsed their TMNT and DC comic stuff. I recently bought a sealed in box Super Powers Batcopter from them, very reasonable price and the condition was nearly flawless, I was extremely happy with it overall. I don't know about the other toy lines you listed but I definitely remember TMNT (I buy those occasionally) so it's worth taking a look.