Any decent art supply store in your area should carry liquid latex. Check the yellow pages and call around. It is usually in the section with the sculpture and molding supplies (most artists use liquid latex to make molds). I generally use Mold Craft Liquid Latex, mostly out of convienence (the local shops carry it) but I am sure there are superior products out there if you ask around.
This is the list of latex suppliers from
The Puppetry Homepage if you still have trouble finding a supplier locally:
Chicago Latex Products
345 E. Terra Cotta
P.O. Box 395
Crystal Lake, IL 60039
815-459-9680
spartan@mc.net
http://user.mc.net/~spartan/cl.htm
Cementex Latex Corp
480 Canal St.
NYC 10013
212-226-5832
Environmental Technologies Inc.
PO BOx 365
Fields Landing, CA 95537
707-443-9323
Heveatex
PO Box 2573
106 Ferry St.
Fall River, MA 02722
508-675-0181
800-922-2933
Wisconsin Latex & Adhesives Co., Inc.
PO Box 04703
1836 South 3rd St.
Milwaukee, WI 53204
14-383-8351
You often have to buy in large quantities from these types of suppliers, but they tend to be much, much cheaper than the average hobby or art supply store in the long run.
You might also want to try finding Tool-Dip or Plasti-Dip. That'll work just as well as latex. It's slightly trickier to work with, but the fumes are not as toxic as latex. I prefer plasti-dip for making teeth, but it's getting harder and harder to find in Canada.
Oh, and if you're searching google specify "art supply" latex. LOL