Is it just the a Mustang or a Mustang +II? I know you said Mustang, but I have to ask. If you're looking for rubberized grips, you're kind of screwed, Pachmayr makes them for the Gov't .380 and Mustang +II but not for the standard Mustang. Your best bet is to get a Hogue or Pachmayr grip sleeve, the smallest size they make.
As for holsters, do you want vertical or canted in the small of the back? Palm in or palm out while drawing? IWB or OWB?