For many years, we have been using Brewers Pitch, one of the traditional a sealants for leather tankards. Unfortunately, the material has recently been withdrawn from the market due to low sales, there aren't as many leather tankard makers as there used to be.
We have considered using one of the other traditional sealants, beeswax, but that comes with it's own set of problems. Firstly, it's very expensive, and secondly, it doesn't last very long. Leather drinking vessels have to be re-beeswaxed (if that's a word) to make sure that the beer only comes out of the top and not out of the bottom. Not exactly an ideal set of circumstances.
Happily, our investigations for a replacement are progressing well. Once we know which of the many possibilities we'll be using, we'll let you know. In the meantime, we have a full stock of every style and colour of leather drinking vessels but, if you want the traditional material, the stock may not last all that long.