How about making your own?
They won't be true turnbuckles.. because they won't have left-handed thread at one end. Tap tubing for the screw size needed. Tak screws and cut the heads somewhat to shape with a dremel.. then flatten a bit, then drill and finsh shaping. You can make a mold and cast JB Weld onto the tapped tubing to get the exact desired shape of the turnbuckles. (assuming its not an open sided one)
You can do variations on this theme to make abut any size turnbuckle you want. Use some Lock-Tite on the screws when you adjust the flying wires. It won't matter that the turnbuckles are right hand thread both ends.