Make one yourself. I built a 3-point tactical style sling that retails for about $40-50 and it only cost me about $7 and 30 minutes of my time. You can buy a kit from strapworks.com that comes with all necessary parts and instructions if you want to make it especially easy. If you want to see some pictures of what I'm talking about and why this style is worth it, then check out this thread at
http://www.ar15.com/forums/topic.html?b=3&f=4&t=233665
Kits can be purchased at
http://www.strapworks.com/category_s/149.htm