I have seen match barrels from Wilson's, Bar-Sto, and Olymic Arms. Been curious myself on how much more accurate you can make the perfect "combat" pistol!
For information or prices check out the following websites:
www.glockmeister.com
www.dennysshooters.com
www.glockworld.com
If anything, I will most likely add the 6 inch factory G20 barrel to my collection.