Anywhere west of I-35 you ought to be able to find Rios.
Oklahoma is somewhat unique in that we have Rios, Easterns, and Merriams. A couple of outfitters here offer what they call the 'Sooner Slam,' where they load you up in a mobile home and take you from the panhandle where we have Merriams to the southeast corner of the state where Easterns are abundant, stopping somehwere along the way to throw lead at a Rio. I always thought that would be a really cool trip with a small group of guys.
That would be an awesome hunt!