I have not been able to find any regular finish preserver--everyone seems to be out until November. I have been using a dash of propane in the can and that seems to work ok.
Recently, I found a cylinder of Genetron 22 at the dump.(chlorodifluoromethane, similar to Freon). Used in air conditioners. It had some pressure still left so I hooked a bit of tubing to it and used that in my Tung oil cans. Don't try it! This stuff is absorbed by the tung oil, leaving a vacuum. I have two deformed cans to prove it.