Where do I find them (web sites).  Is there anything special I should look for?

Check Oly arms.  They offer the best setup for the money.  Colt and ASA make them but they require modified Uzi mags.  The oly upper with the SOCOM block allows you to use Sten Mags which are 4 bucks a pop.  The mag block is 62.50 shipped.  

Plus for the oly upper there are no other mods, just remove your 223 upper and replace it with the 9mm and slap in the magblock and you are ready to rock.

Also look at the forum for pistol and rimfire forums.

Hope this helps

max

I just ordered an Oly 16" 9mm upper with set screw type compinsater and 1 modified sten mag on 28 Dec for $639.00 shipped. Will order the SoCom mag block this week. I shopped around for months for a good used one, then finally took the plunge. Oly said since 9/11 they can't keep them in stock,all are being shipped as soon as they are built/test fired. From what I can find Oly makes some of the best 9mm's for sale. I've also heard that Rock River Arms will be in production soon. Hope this helped and good luck.

Thumper

Thanks Guys, I will look into the Oly stuff.

I just purchased a used Oly setup with the setscrew compensator from someone on this board.  Fires great.  Just waiting to go to the range again.  Winchester 9mm 100 rds = $11.  And I can take it to the only range in town (which only allows pistol caliber ammo.)

However, since I am a lefty, it's spitting at me when it ejects.  Not really the brass, but gunpowder, I think.  

I've seen the larger deflector on the Colt 9mm.  Can you buy that and attach it to another upper, or is it specially pinned on or something?