With something that simple, you don't buy it, you build it.
Find or buy a short bolt with the correct thread to fit into the drain hole. It can be steel or brass; I used steel. Go to a model airplane hobby store, and buy a short length of brass tubing that is small enough to go through the bolt. Those hobby shops also carry many sizes of plastic tubing. With your brass tubing in your hand, find a matching size of plastic tubing that will fit tightly, as you slide the plastic tubing over the brass tubing. Buy a short length of that plastic tubing.
Use a micrometer, a vernier, or a ruler, to measure the OD of the brass tubing. Drill a hole, same size as the brass tubing, through the drain plug. Push the brass tubing through the hole you just drilled in the drain plug. I think I used 3/16" brass tubing.
Its simple to make.....................easier if you have a drill press. I had the parts in my spare parts boxes.