I checked online everywhere and could not find anywhere that sells the painting you are talking about. I did, however, find which painting you were talking about, I think. It sounds like it was a painting made for a specific purpose and they didn't make duplications to sell to just anyone.
A lot of the Chinese art shops sell this statue of him though that I found. The web address is in the Source section. It's a pretty cool statue if you are looking to collect Kuan Kung items.
