Try looking at Craigslist.org, for the Minneapolis area. There are plenty of CB750's listed. You can also check out the surrounding regions (De Moines, IA; Madison, WI; etc.), mostly the areas South of Minneapolis, if you are willing to drive that far.
Pull out a map of the states surrounding your area, and look up the metropolitan areas. You should be able to find something within your driving range. Just be wary of showing up in a strange area, alone, with a wad of cash in your pocket... Be safe! Ask the owner to drive the bike to a public place to close the deal. Best of luck! Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII