Congrats! I'm in the process of finishing my first 6.8SPC build. I'm also researching barrels as I've done a bit of homework and seems I have more questions now than I did when I started. Anyway, here is the 18" barrel I'm looking at from Noveske Rifleworks:

http://noveskerifleworks.com/cgi-bin...since=&status=

MidwayUSA carries this barrel on their website and you can even get it with a headspaced bolt from them for a little extra. If you are thinking about going with a suppressed 6.8, I would highly recommend getting this barrel directly from Noveske with the switchblock option as it will allow you to adjust gas pressure.

Cheers!