do you have the MOE version, or the standard version of the 15-22?
the MOE version has QD swivel mounts in the stock and the rear of the reciever.
the standard version only has the mounting locations in the M4 waffle stock.
i got one of these Troy sling mounts from AIM $27 with free shipping.
http://www.aimsurplus.com/product.aspx?item=XTSMOUCOM00BT&name=Troy+Industries+Rail+Mountable+Sling+Combo+Mount&groupid=85
it's got the hole for the QD sling swivel, but i'm just running a standard GI "silent sling" which was about $5 from the local shop.
AIM has other sling mounts that don't have the loop, just the QD mount so if you have the MOE version you could add one QD point in front.
but there's a boatload of "tac slings" out there. it's a matter of what style you like, some have just too much webbing IMHO and are like fucking an octopus.
unfortunately, S&W didn't make something like this in polymer, just for a web sling (non QD) and include it with the rifle.