Used bike, used bike, used bike. Mountain bikes from the mid-90's with no shocks and a cro-mo (steel) frame are good bets. Their geometry is similar to touring bikes, with straight bars. Look for ones that have named components (ie Deore) rather than ones that are numbered. Usually indicates higher grade of components (unless it's Tourney.) Some have braze-ons (holes) for mounting racks. SO has a 1995 Norco Tango that makes a good commuter. Trade in the knobbly tires for smoother ones.