Can anyone point me to a website that sells the sling used by the Marines on M16A4's in Iraq and Afgahnistan right now? I was thinking they used something like the Giles one?

There is no single sling in use.  The sling issued from the armory with the weapon is still the same old design.  M1 Garand & AR-15 Sling, seventh up from the bottom

But, many units use discretionary funds to open purchse various types of slings for their Marines, and many Marines will use their own money or ingenuity to acquire various slings as well.   You name it, some Marine somewhere probably has one.

Being on the ground here, the answer depends on the unit and the availability in the front line BX/PX operated by AAFES.  This is a result of some units ordering GSA or off the shelf items for all their troops and the guys get to use them during training before arrival.  This is of course the best case and limits the sling type to a single type in each of those units.  

The other option where the Marine or Soldier goes into the BX/PX and selects one of the few types they have there.  The carry CQB style slings that are specifically marked for M-16 or M-4.  I have seen SPECOPS  Brand Mambo going for $24.95 and and eagle industries TAS-3 going for $17.98
I went with the TAS-3 and it is well made and works as designed.  Some of the unit purchased slings are Blackhawk from BOTACH http://www.blackhawk.com/product_detail.asp?product_id=1785&d=1  These are available to the unit for $29.96.  I think I got the best deal so far and am pleased with it.

The Army Owns the Night
God bless America
Very Respectfully,
David

As SSeric02 stated, the MarCorpSysCom approved Combat Assault Sling has yet to be fielded, last I heard there were like 3 contenders, but I cannot remember the names of the manufactures (they weren't the usuals suspects like Eagle, etc last time I heard).  Most units use 1106/1107 funds to buy COT slings, some good and some absolute crap.  When the A4 MWS shows up from the depot, it has a "silent sling" with it that really cannot even be used on the KD, so units will have to pull the snap version off their old A2 to use those.