Should be fairly common but Walmart for sure carries it in both spray and brush on forms.
Never had an issue with water raising the grain and washing it off. These old stocks have been out in the rain, out in the jungles, in the mud, etc. Considering they are saturated with BLOor cosmoline
, water doesn't penetrate far. Mine usually are dry in an hour.
Tung oil may be a better finish but it isn't appropriate for military stocks. Not saying you can't use it but if you want to be accurate, BLOis pretty much it for most stocks.