[villagetinker]

We have 2, for our cats, one that fit in the sliders, and one that was just built into the lanai enclosure, both of these were available from Lowes, but were purchased there, these were picked up from neighbors that no longer needed them. If you have a real door (wood, metal, fiberglass) these are not hard to install if you have the correct tools, IF you have a GLASS slider or similar door, you can NOT install a doggie door in the slider, you can buy a panel that can be inserted in the slider track, and they work well. You can also purchase supplemental locks to keep others out of your house.
Now a word of caution, we only let out pets out to the lanai or enclosed birdcage, we would never let them out to an open area, too many predators in the area.