So far most everything I've seen has been 9mm. While fun and cheap to shoot, I personally would prefer a .45.

Any leads?

.45 is more expensive to shoot than .223 sometimes. hehe

olympic arms is your choice if you want to go that route..or even 10mm.

Auto Ordinance "tommy gun" now owned by Kahr Arms.  It be next on my list.   30 round sticks for $19.99 at CDNN and the 50 round drums.........
linky

Oly .45 is a good choice.  I have blasted thousands of rounds through mine.

Kurts Kustom firearms
Look in the industry section

Gracias Dave (cute kid too)

Quoted: img.photobucket.com/albums/v443/superdavarfcom/DSC00016.jpg Kurts Kustom firearms Look in the industry section

caption:
"...What do you MEAN I can't have a cookie??? I suggest you re-think that decision..."

The Oly .45 uppers are pretty pricey, around $500 when I bought mine and the mags are also on the expensive side. There are mag blocks that work with the CavArms MKII lower that let you use GG mags which are way cheaper than the modified uzi mags that the Oly Uses. There is also a way to modify a Hahn 9mm mag block to fit .45 mags (takes a bit of filing) that allows you to use slightly modified uzi mags. The only real issue with the .45 system besides the price of ammo, is the expensive magazines.  Not to many places are making stuff for the .45 AR so the pickings are slim. I love mine and its a blast to shoot.