Matte Paintings - where can I purchase?

While movie props galore can be found at auctions houses and eBay, I have rarely seen matte painting for sale or auction. At Comic Con there are a couple of vendors that sell paintings and production cells from animated shows, but I haven't come across any matte paintings. Does anyone know of a dealer/shop or auction house that sells matte paintings?

In my experiance few and far between shipping is a nightmare