From: Rubber The Right Way - A Condensed Primer on Making Your Own Latex Clothing
(not an exhaustive list and not an endorsement of product)
Some garment manufacturers sell latex by yard/meter. Prices are double what you would pay directly, but you can purchase small quantities this way.
Some places make their own latex sheeting in short runs. Cottage industry latex is often sold in sheets instead of rolls and can be very high quality material. Some sheet latex comes with a peel away backing, useful for tracing and cutting pattern pieces without clean up of markings afterwards.
ex. Latex-U-Wear (http://latex-u-wear.com)
Industrial rubber suppliers often carry latex sheeting in limited colors (usually semi-transparent and black) but can often provide unusual widths and thickensses. Quality can vary considerably.
Industrial suppliers often carry other kinds of rubber that may be of interest.
ex. Canal Rubber (www.canalrubber.com)
Theatrical supply houses often carry semi-transparent latex sheeting as well. Once again, quality varies considerably.
Theatrical supply houses also stock latex dyes and other types of rubber for creative rubberists.