Superscope is the machine that you are looking for. You can get it at either the Jamie Abersold Jazz site or do a search for Superscope. They are expensive but they do the job.

i have been looking for a machine for this very purpose and this is the only one I have come up with. i got an ad in the mail for some other brand but tossed it out unthinkingly. i wanted to slow down jazz solos for the purpose of transcription.

Good Luck