I picked up some NOS (new old stock) cables on eBay, but it took some patience and a careful eye to spot the right ones. In the end, though, I wound up replacing the drivetrain with Shimano parts because it turns out the freewheel needed replacing, too. SunTour Accushift freewheels don't use Shimano-compatible spacing and replacement Accushift freewheels tend to be more expensive than Shimano 7-speed shifters plus derailleurs plus freewheel.

I gave away most of the usable SunTour parts from that '91 Cannondale, but I might still have a cable or two. I'll check, and if I do, I'd be happy to send them your way.