AS PE has mentioned here, most film processing amchines used for Movies are high speed, and the labs tend to use splices that would mess up several frames at each end of a still roll. Several small Independant labs used to handle stills as a way to increse their business, Folks Like RGB in Hollywood, but many of them have closed.
I think the last one standing was Dale Labs in Florida, and I almost missed out, sending them some rolls and finding a note in the return package informing me that the service was being discontinued at the end of THAT month.