Where would they get DC-9-21's from? According to Bill Harms website there is only one flying -21 today belonging to Allegiant Air, and it's regeistered N127NK.
The other ones are out of sevice or scrapped for parts. The -21 was a limited series, don't know if anyone except SAS ordered it. It think it had the body from the -10, the wing from -30 and the engines of -40 or something like that and was used to serve the high short runways in northern Norway. The last one left SAS some time ago.
Just like birdwatching - without having to be so damned quiet!