OMCs been out of the stern drive business since the mid 90s. On the Cobra's parts are still pretty available, but like racerone said sometimes you can't find anyone to work on 'em. Merc and Volvo are more popular.
I'm not quite following the description of what's wrong. When out of gear the prop should spin pretty freely. If in reverse, you're not spinning with the shaft ... it's broken. Could be a couple of different things. Until I knew otherwise, I'd assume it's the expensive things ... like new lower unit.