Looking to build a long range rifle using the .308 cartridge, built for accuracy on the AR-10 action, but have never seen parts kits for sale, nor 24" bull barrels.

Can anyone point me in the right direction?

This will be for varminting, hunting and paper punching, so weight won't be an issue. It will be scoped with a high power adjustable scope as well.

the ar-10 is too new a design to have parts kits, new barels and such for it (at reasonable prices anyway).  the best bet is to get a target upper from
armalite.  

Lowers are available from Armalite (Eagle) as well as all the other parts.  After market triggers will work for the most part.  The Jewell does fit and work but Jewell won't admit to it.  Just about any body will make a barrel for you.  Look here: www.shooterready.com/barrels  or here.  This fellow is making me a barrel for mine www.mikesshooters.com.    JP Enterprise, gunsmokeinc,  Wilson Combat,  etc, etc.

Upper receivers are pretty much only available from Armalite, as well as all the associated parts but if you want a Kreiger or Douglas barrel you pretty much have to go after market.  I'm trying a Lilja 3 groove from Mike Orwan.  He also makes and adjustable gas block/gas bleed for it so you can tune it to bullet weight and function.