Installing wall mounted vanity

I want to seal the holes where the pex comes from underneath the crawl space up through the floors to my fixtures. I want to stop the draft coming through. The manufacturers instruction say to only use water soluble gypsum based caulking because others might react with the pex. I cannot find anything like that around here and people I talk to that install pex have no clue when I mention water soluble gypsum based caulking. What do people use?