Nevermind, I think I know what you're talking about. Not all saddles have those. In all honesty, you don't need one, and it's not great for your cinch to be kept on your saddle all the time - I take mine off and rinse it after every ride. I don't use a traditional rope cinch because they tend to rub my horses. I use a ClassicEquine Soft Touch cinch. I like them MUCH better.
But if you want the piece for storing your rope cinch with the saddle, your local leather craftsman can help you I'm sure. Shouldn't be too expensive. You won't be able to find one at a store or anything, they don't usually sell parts like that.