You can make your own. Otherwise, I don't know. To get long storage life in a liquid stock with ascorbic or isoascorbic acid probably requires an organic solvent like a glycol that doesn't ionize until you add water. If you can get an organic "sulfur dioxide complex" such as is in HC110, you might get a satisfactory single solution stock that simulates Xtol. The article by Jordan Wosnick that is listed in the sidebar tells how to do it in two solutions, one in glycol, the other in water.