Looking for a 556/223 20" barrel for my rifle with 5/8-24 threaded muzzle?
Any info on where to find one?

What caliber?

I've seen some Grendel barrels with 3/4 threads but your best bet would be contacting someone who can cut and thread it from a longer barrel.

Buy a non threaded 20" barrel, then have ADCO thread for you.

If you're looking for truly high quality, Compass Lake has several options.

They do Douglas and Krieger barrels, three different profiles, your choice of muzzle thread, finish, different twist rates, their own chamber too, you can get a matched bolt, etc.

They're some of the most expensive barrels I've ever seen, but they use high quality blanks and they're very highly regarded. I don't remember how I stumbled on their site, it might have been while I was reading one of the SPR threads.

Buy a barrel of your choice and send it here.

http://www.ar15.com/forums/t_7_143/1029861_Barrel-Threading-sale--50--PTR-91--Tavors--Augs--Scar--Sig-MPX--Beretta-21A-Threading--FAL--PAPM92.html