I want to put a side-mounted sling swivel on my CMMG MOD4.  I like the Colt style or RRA style (it might be the same thing.)  The "Sling-Thing" is an acceptable alternative, but I would rather find one that attaches to the barrel instead of the FSB.  

Where can I find one?

Fulton Armory, Bushmaster, DPMS,....  The question is "who doesn't carry at least one?".

Honestly I like the Fulton Armory ones better than the ones from Bushy.  They are very similar but the Fulton ones seem to be made 'heavier duty' (more like the Colt version).

I was just looking for the same thing.  Bushmaster sells 2 on their site, Bravo Company USA sells one, RRA has one on their site, and I ended up getting one from Fulton Armory.  It isn't advertised as something for sale but is pictured on alot of their custom uppers so I called them.  For $25 plus shipping I got an all-metal side swivel kit that mounts under the FSB with the rollpins.  The finish matched my 6520 nicely, and I am very happy with it.  

If you want one that clamps to the barrel then the best I've seen is the RRA model that is sold by at least a few vendors who support this board.  

That's the one!  Thanks!  I'll check out the supporting vendors, first.

I just ordered one from ADCO for $23!  Bushmaster was selling theirs' for $35.