I would take Michael's and Prism's thoughts with a grain of salt.
IMO, Prism is way too small to compete in the film sales arena. Even 10 years ago, their range and stock was small.
And, Michael's is just retail burglary. Not only are their prices absolute bullshit, I feel that I need a shower after every visit there.
Vanbar might be a better source; however, I have no doubt that two areas eclipse retail sales: eBay and off-shore purchases. Obviously, I have no way of knowing volumes, but most of the film users I speak with primarily buy from these latter sources.