From one Big Red Machine fan to another...
You should be able to find a Bench-endorsed mitt without too much trouble. They were produced in great numbers and, even though it's been a few years since the 70's and 80's, they aren't exactly rare like some early turn-of-the-century antique gloves.
My first suggestion would be to keep an eye on eBay. There are almost always a handful of these up for sale at any given time.
There's also a good chance somebody from this forum has one available and maybe they will offer one up. If not here, try posting in the "Gloves Wanted" section of this discussion board, which is dedicated to more modern gloves:
http://glove-works.getforum.org/board/
One more thing to check out: I recall a thread here back awhile ago about the subject of various Johnny Bench model mitts. Try the "Search" function and you should be able to find it. If I remember, that thread had a complete list of all the different Bench models issued by Rawlings.