Umarex makes G19 and G17 airsoft and BB. I had the 19 BB model. The 21 is an slightly up sized G17. Although the G19 was a extremly close replica it was slightly larger and would not fit any G19 holsters I have, it may fit G21 holsters. The slide is slightly wider than the G19 so size wise it may be closer to a G21.
I really liked it for shooting at a BB trap in the garage unfortunatly after a year or so and several thousand shots it began leaking CO2 and Umarex offers no service for them so it is a throw away if it has problems. Sometimes they can be found at Acadmey and Cabale's.