I'd be very surprised if the rigging has stretched. I think it's much more likely that the rest of the boat is moving, for instance, the mast step getting soft, or the cabin roof is rising and pulling the topsides in, or the tangs on the mast are working downward.
Shortening the studs on the turnbuckles is a perfectly fine fix. Just make sure that the remaining engaged thread length is at least twice the thread diameter (e.g., 1" minimum engaged length on 1/2" diameter threads).
By "traditional", do you mean galvanized steel wire and turnbuckles? If so, shop around. I think you'll be pleasantly surprised on prices. For instance, look at McMaster-Carr (McMaster-Carr Supply Company
. Search for "turnbuckle". They carry a lot of parts in forged galvanized steel. Stay away from hardware store products because a lot of Asian pure crap is being sold. McMaster-Carr is an industrial distributor with no minimum order, so they carry decent quality. They also carry galvanized wire rope and other fittings. Figure out what you've got now and you can probably replace it all, and for far less than you've been quoted for stainless rigging.
When you do unstep the mast, I'd make a very careful inspection and not stop until the source of these loosened shrouds is discovered and fixed.