You can probably get a pet door at your local pet supply store. But before you do, I encourage you to look at some websites and acquaint yourself with the various options.
Some work with regular doors.
Some work with windows.
Some work with sash windows.
Some work with sliding glass or patio doors.
Some work with screens.
And if you want to get fancy, there are motorized, electronic, infrared and magnetized ones.
If you are Rubenesque, you might want to check the sizing recommendations and stick to doors made for larger cats.
I encourage you to take a look at the following websites and most especially, to have your folks carefully read ALL the associated information. Careful reading will put you in the best position to shop wisely and well.
Patio Pacific. This site also has a good section on getting you used to using the door. See Training Your Pet, at the top of the site's main page.
http://www.petdoors.com/
Pet Doors, USA
http://www.dogdoors.com/
The good news is that there is lots of variety. And many have thermal properties suited to cold climates (if that is where you live). Some have impressive security features. I could go on and on.
Using Your Pet Door
Obviously if you have splurged your life savings on an electronic door that opens for you just like the automatic doors at the grocery store, then you need read no further. But let's face it. For most of us, a relatively inexpensive manually-operated (by us, that is) door will do. This means that most of us are stuck with the door we butt open with our head and which then trails along our back as we push through. Trust me, you will get used to the feeling.
Most of us can easily be trained to walk through the door, even if we are initially reluctant to do so. At first, your purrson should open the flap and allow you to get used to walking through it (with it being fully opened all the time). This means that the purrson will either have to prop it up or hold it for you.
Once you are used to that, then you need to get used to going through the door, starting from its closed position. Your purrson needs to make it worth your while - either by having a tasty treat or tempting toy on the other side, or by pulling a string toy from one side of the door through to the other so that you will follow it.
And don't forget, the Patio Pacific website has more tips on getting you to use the cat door.
If that absolutely doesn't work for you, you can consider the more expensive Plexidoor (available through Pet Doors USA) which opens at the side (like a regular human door) rather than at the bottom.
