I bought mine off Ebay for like 6 bucks, but you have to put split key-ring rings in them that aren't screen accurate. Search for rifle sling mounts and you'll get lots of results.
I am working on getting some together and am planning on doing a run in about a month or two, if my supplies come through. If you can wait til then...I wont be using key rings though, itll be much more screen accurate