Montanan,
The mdl. 20 should be about the same price as the mdl 21,and I'm sure they can order it for you. Don't be detered by salespersons that just want to sell what they stock, know little about guns, and nothing at all about your preferences.While the ammo for the 10mm is not as common as the 40/45, it is available. You just have to look for it. If you want full power ammo you can load it yourself or purchase it from several sources. If you are going to shoot IPSC the mdl20 with the .40 barrel would make an excellent platform. You can make power factor with ease with a low recoiling weapon. There is lots of inexpensive .40 ammo out there, and you reload as well, so ammo would not be a problem. I currently own two Glocks in .45, the mdl 36 and 21. Don't currently own a .40, but for a reloader brass and bullets are no problem. The brass is everywhere I look! Get what you want! Don't compromise! You'll be disappointed with anything else!!
Stay Safe,
Savage