Rifle Sling

Where can a get a rifle sling like I used in the Marines for my AR 15?

Not sure exactly which sling you used, but here:

http://www.charleyssurplus.com/army_military_field_gear/slings

https://www.libertytreecollectors.com/productcart/pc/viewcategories.asp?idCategory=54

I use an M1 sling on my AR. M16 web slings with the compression lock are longer, but it makes little difference.

A “modern” 3-point sling: http://www.slingsonly.com/m16_tactical_sling.shtml

Improving your shooting #7. Using a leather rifle sling.