What is wrong with it? I recently sent my P6 to http://www.baierfoto.de/. Rolf Baier will overhaul the winding mechanism and install a modification of an external indicator to show when the film is correctly advanced, he also offers a shutter overhaul service.
The two services together will probably cost you more than the purchase of another body, but for me it was worth it to have a reliable camera rather than another unknown.
There is of course eBay for purchase of Pentacon Sixes. When you do a search you'll need to wade through all the auctions for lenses, other accessories, and even the occasional Kiev 6C or 60, though, since sellers seem to include "Pentacon Six" in auction titles for these items.
I have been thinking (and still do), about sending it to a repair shop. Regarding eBay....well I might try to buy
one there, but there is just to much junk there. If buying I prefer to get one from another enthusiast.
Just out of curiosity, I checked KEH. They've got a Pentacon Six (presumably only the body, although that detail is unclear), in "EX" condition, for $41: Dead Link Removed
The winding mechanism is the usual problem. When I had one, I remember this was very fickle. If Rolf Baier's modification is a definitive fix, then it sounds like something worth looking into.
I have one of these cameras myself. My opinion would be to have your camera serviced and adjusted. Almost any used Pentacon 6 is going to have (now or later) "issues" that will need to be addressed, as they are not very robust. If you have yours serviced, then you "KNOW" you have a good working camera.