Look Through online LARP shops. "Live Action Role Players" use liquid Latex and foam material to make their game weapons. Some of them offer the stuff for do-it-yourself weapon"smiths".
http://funadvice.com/r/15vjdrfdh70
Carpet stores keep it on hand. If you just need a little liquid latex bring a small jar to a carpet store and ask if you can buy a little from their supply. If you need a lot they can order larger quantities for you.