Picking leather for a strop.

So I have struck up a bit of a rapport with a leather worker at a local flea market. As I was talking to him about the weather and whatnot we got on the topic of straights. He used to shave with a straight, and understood the ins and outs of using a stop with and without paste ans also using linen. We spoke for a bit about him making a strop for me, and left on the terms that I would figure out what I wanted and get back to him. He will let me take a piece of leather with me to try, and I wanted to know what I should ask him for. I am not sure if he really has any higher end stuff, as in Cordovan or any of the other top dollar cuts of hide. But as far as general day to day readily available leathers go, what should I request from him? Any particular piece of the hide? What thickness would you suggest? Width? Length? Is horse hide better than cow? Just a few questions before I start having him work on it. I have a strop now that is really slick, and I am not sure that it is doing any good whatsoever. I also have an old lumpy loom strop, and plan to make a large loom if this little test pans out.