http://www.under-tec.com/store.php

My bad. Someone does make underwear… Under-Ease are underwear for protection against bad human gas (malodorous flatus) and are made from a soft air-tight fabric (polyurethane-coated nylon). To maintain the air-tightness, elastic is sewn into the material around the waist and both legs.
A triangular “exit hole” for the flatus to be expelled is cut from the back of the air-tight underwear, near the bottom. This “exit hole” is covered with a “pocket” made of ordinary porous fabric sewn over the “exit hole”. This unique design forces all expelled gas (flatus) out through the “pocket”.  Inside the “pocket” is a high-functioning, replaceable filter - the core of the technology. This multi-layered filter is made in a sandwich-style, and begins with the two outer layers of wool felt. The second two layers are made of non-woven polypropylene and spun glass materials. In the center of the filter is a single layer of activated carbon.  The filter is then covered with soft ordinary material to allow for easy replacement in or out of the pocket. The underwear are washable and will last approximately a year depending on the frequency of use and laundering. Each filter will last from several weeks to several months depending on the frequency of use and laundering.