Anyone know of one?

There are several made, but they are NOT recommended for use in home defense OR sporting use.

The weight of the shells makes the sling pull the gun around as you swing it.  Swing the gun on target, and the sling continues to swing pulling the gun off target.
Then it swings back, and pulls the gun off target again.

Uncle Mike's make one:
www.unclemikes.com/adtemplate.asp?invky=9006032&catky=1279575&subcatky1=4548835&subcatky2=3850977

Blackhawk makes a good quality one:
www.blackhawk.com/product_detail.asp?product_id=1774&d=

I'm sure there are others.

I bought a Blackhawk one and it's a piece of crap.  Can't really use it for 12 gauge because the shell loops are really tight, if you put all 15 shells in, the sling will curl up.  

I left shells in it for like a year, hoping that would break it in.  Never did.