I don't remember the brand of the one I got, but they come in different sizes. I like it that I can set the door to swing both ways, let cats in only or out only, or lock completely.
You can get more expensive ones that open in response to a chip in the cat's collar. The big problem with that is, the cat has to keep the collar on. Mine never will.
The instructions said to install it in the middle of the door, but I put mine in the corner opposite the door handle. My sister and BIL had some stuff stolen when somebody reached through the cat door and unlocked their door from the inside. (The cat door was there when the moved in. They sealed it up afterward.)
I got mine at either Lowe's or Home Depot, don't remember which. They had quite a few different ones with samples out so you could see how they work and how big they are.
