Compaq used to make a nylon belt case that would fit that bill (it was big enough to fit the iPaq with a dual PCMCIA card sleeve), but when I ordered mine they said they were discontinued and I got a Targus POS that looks similar, but 1) is smaller; and 2) doesn't have a full strip of velcro to accomodate different sized loads (iPaq, iPaq with sleeve, etc.)
Will see if I can find one on eBay to show you.