Last time I needed to buy a tire on the road (front) I found one at a small indy dealer about 30 miles south of my then present location. Road down there, pulled the front wheel, and they swapped it out and balanced it for free. Hard to beat that deal.
Another possible source of a 160/70/17 rear is yur local Harley dealer. Some models of Harley use that size. You can get an old style Dunlop K591 with "Harley Davidson" molded into the sidewall. Prolly won't be cheap, but what the hey!