Howdy Mr. Matt,
Count me as another 357 Sig fan. I currently have barrels that allow me to shoot the round from both my Sig P226R and P229, my FN HP 40, and recently received a Bar Sto to use in my CZ75B 40. I do think the round has a lot going for it. I also would echo the warning from Mr. Chris - the round can be very addictive, especially if you reload.
As far as a barrel source, you might want to check out EFK Fire Dragon. Their website will probably list both the 357 Sig barrel and the 9mm down/conversion barrel (which I think is what you are asking about) for the Glock.
I have their 9mm down barrel for my P226R (purchased in 357 Sig) and am using their 357 Sig barrel in my HP 40. Their barrels are designed to be a true drop in and should not require any fitting either. Unfortunately I ordered the semi-drop in Bar-Sto which usually does not require any fitting, but it will take quite a bit to have it function properly according to my CZ gunsmith. So when I can purchase a barrel from EFK, I tend to use them first. Theirs are drop in units, more accurate than I can be in my opinon, and cheaper than most other barrel companies too. So to me they are a real bargain, especially since I have never had to have one fitted to the pistol it was designed for.
If you will check various rooms here, including reloading and ammo, you should find several posts on the 357 Sig caliber. But if you still have any questions, please feel free to ask them.
Hope this hopes a little. You could also check the Bar-Sto website as I suspect they will also have these Glock barrels as well.
Just a cautionary note to you and other members: on the 9mm barrel for the Glock 23 (and other pistols), make sure you order the 9mm down/conversion barrel and not a standard 9mm barrel. The conversion barrels are made to the same outer diameter as the 40 and 357 Sig and will fit your slide properly. A standard 9mm barrel would be too small to work.