Mt Spokane Photography said:
I think that you can find used ones on ebay and even Craigslist. I see a local is asking $44 for one.
The issue isn't availability, RRS still sells the original design plates for the 5DIII. What the OP is looking for is a plate with the QD socket that RRS has designed into their newer plates, which is the connection point for their in-licensed strap attachment system (Magpul, a company that makes carrying straps for firearms).
I really doubt that RRS will go back and re-design older plates to include the socket.
Jim's solution is what I use – a Kirk 1" clamp attached (with Loctite) to the lug of my blackrapid straps. I prefer that, because it allows me to connect the strap to any Arca-Swiss dovetail (camera or lens plate).