It's called TTC (Total Taxidermy Care) and you can buy it from Bringmin.com
Traditional taxidermists don't like the stuff very much because it dries extremely well in places. The biggest complaint they have shared with me is the critical facial structure. The TTC can jerky critical parts of the lips and eyes that can reallt **** up a mount.
So, what I've learned from my own experience is to do a really thorough job fleshing before any salt is applied (2 days after kill i might be ready). Use TTC on the made body of the cape and stop at the neck line, then ONLY table salt on the face and ears. This usually requires about 5-lbs of table salt (1 gal juice jug). Personally I wouldn't use it but as a last resort. Never use it on unfleshed hides because it'll cost more in some cases for difficult prep work.
If you learn to flesh thoroughly, very little salt is needed to stop the hair from slipping, which is enough to get you out of the field to a ready supply of salt to finish the job for your taxidermy hand/off.