Yes, there should be Parks rentals available at the International Gateway entrance.
If you are considering renting for more than a couple of days, you will most likely save money by renting from an outside vendor - not to mention it gives you the use of the
scooter at the Resort hotels (which, as you know, can feel twice as huge when you are in pain) and at Disney Springs, which is just *so* much bigger than it used it to be! (Or, who knows... maybe it isn't really bigger... it just feels that way, like how when you fall, the ground gets harder and further away with every year that passes!)
You can also consider other solutions, though. Some folks don't really need a scooter as much as they need a place to sit, and in that case, you can rent a wheelchair, and just push it around until you need It. Main thing there is that if you will need to sit while it is moving, you will need to have someone with you who can push you - most folks are *not* ready to self-propel at WDW unless they do it in their daily lives.
A rollator is another idea - it also gives you a seat when you need it, and something to hang on to while you walk. You can rent rollators from outside vendors, but it might be cheaper to just buy one outright; basic models can start around the $65 area on
Amazon.
Last but not least, remember that airports can be HUGE - with long concourses, so plan accordingly if you are flying into Orlando. Wheelchairs, scooters, rollators, walkers, etc. all fly for free on all US domestic airlines as medical equipment, and will be gate-checked for you by the airline.
Have a great trip!