Its pretty easy to find bits to make a tumbler, see this thread http://aussiepinball.com/index.php?topic=1373.0 for what i made a while ago.  It was a pretty fun little project - the only thing is over time, the small fan motor burnt out - was probably waaay to small for the job.

As others have said, the idea is the media and parts 'tumble' (surprisingly enough) over each other, so there is probably a range of speeds this can be acheived