The following forums (or conversation boards) either specialize in mouseguns, or represent manufacturers that include mouseguns in their lineup.

Beretta (Beretta makes small .22, .25 and .32 caliber pistols, as well as larger guns.)
Bersa Chat (New!)
Bodyguard 380 (S&W) Blog and Forums
Charter Arms Lightweight Revolvers
Elsie Pea (Ruger LCP) Forum
Glock Talk -- The G26(9mm) and G27(40S&W) are Glock's lightest firearms.
Kahr P380 Forums
Kahr Discussion at the Handguns Forum
Kahr Club on Glock Talk (Kahr makes .380 and 9mm mouseguns.)
KTOG (Kel-Tec Owners Group)
KTRange DOT COM (Kel-Tec Firearms)
Micro Desert Eagle (Discuss On GlockTalk Forum)
North American Arms (NAA -- very small revolvers and pistols)
P380 (Kahr) Forums
Rohrbaugh Forum The R9 is the smallest 9mm pistol.
Ruger LCP Forum
Ruger Mousegun Revolvers
Seecamp Forum (Started the pocket gun mania with the Seecamp 32
Smith and Wesson (S & W makes some very light weight revolvers)
Taurus Light Weight Revolvers and Semi-auto Pistols
USRange (Has Areas for Kel-Tec, Kahr, J-Frames and Others)