The following forums (or conversation boards) either specialize in mouseguns (small light-weight firearms weighing under 16 ounces), or represent manufacturers that include mouseguns in their lineup.

Beretta (Beretta makes small .22, .25 and .32 caliber pistols, as well as larger guns.)
Glock Talk -- The G26 (9mm) is Glock's lightest firearm.
Kahr Club on Glock Talk (Kahr makes the PM9, a very small 9mm pistol.)
KTOG (Kel-Tec Owners Group)
KTRange (Kel-Tec firearms)
North American Arms (NAA -- very small revolvers and pistols)
Rohrbaugh Forum The R9 is the smallest 9mm pistol.
Seecamp Forum (Started the pocket gun mania with the Seecamp 32
Smith and Wesson (S & W makes some very light weight revolvers)