If you are doing ESB... you could just go to the home depot and get acouple of small brass eye-screws, than get two small (3/4") metal rings... or you could even use some small key-rings. Screw the eye-screws into the stock and grip, than put a metal ring onto each of the eyes.
As far as the sling it self.... just dig around at the good-will, find a skinny old leather belt and cut it up.
Anyway, that would be a cheap way of doing it.... not what I personaly did... but still an option.
I just bought some D-ring clasps which have two screw holes each to attach to the stock with. I bought some scrap leather from Fabric Depot and I am in the process of making the strap so that it is strong enough to sling the blaster. I'll post/send pictures when I'm done if anyone's interested.