Creedmoor is probably the best place to find them. You can get them at the CMP too. You can also do something similar with a cheap USGI web sling if you don't want to spend money for the fancier leather one.
We did a lot of shooting like this back when I was doing the Appleseed thing a few years ago. You can use one with an AK, but it doesn't work quite as well. Especially if you use the existing front sling mounts on the side of the gas block. It's definitely better than shooting without a sling, but there is a tendency for it to cant things a bit. I found it works best with rifles that have the sling mounts on the barrel and on the underside of the buttstock, like the M1, M1A, AR, or FAL.