This was a blast from the past, haha! Had to look because I couldn't remember this and realized it was five years ago, lol. Since then, I've found more, and better, info on the creation of the breed. And they did in fact use RIR and Plymouth rocks to create them :) Or at least the info I've...
So here's what I found. A brief mention of oklabars here: https://digital.library.unt.edu/ark:/67531/metadc5993/m1/61/?q=oklabar
And this one talks a little about the 'recipe' for them...
Let me dig it up. The info I found sounds like it was a BR male over RIR female, and then taking the barred birds from that and backcrossing to RIR, basically the same thing that was done to make the campbar.
From my searching, I didn't come up with any breeders or place you can get them. They are however from RIRs crossed with barred rocks. If you really want the breed you could attempt to 'recreate' it by doing the crossing yourself.