I got a huge python skin from Bernie Shrunk a few years back - he got it from a neighbor who's pet croaked. I have used it on a couple bows. It is as thick as deer rawhide and I use it when backing is indicated. There are probably 6 bows worth in the one skin. Bernie dried it as rawhide and not tanned like most of the python skins commercially available.
Mike at Pine Hollow sells boa skins (this is what Brad Merkel has used) and I have a couple from Mike. They are thinner than python but still work as backing - much thicker than rattlers. One boa skin will do at least one bow, maybe 2.