All of the places that Eeyore1954 posted have been given good reports by DIS posters over the years. They will all deliver to your resort; Randy's Mobility requires that you be there when they deliver and pick up. The others will deliver and pick up from the resort whether you are there or not.
Even if you plan to get a wheelchair at your resort, you should probably take the numbers for the off-site rental places with you. There are a few
buts involved in getting a wheelchair from your resort that DISer have posted about over the years.
The resorts have not in the past allowed people to reserve a wheelchair in advance, so some people have found that the resort did not have any wheelchairs when they got there. Or, the wheelchair that was available was not right for them (not in the best condition or it was extra wide when they needed small or vise versa). They can request a different wheelchair from another resort, but people have posted that can take anywhere from a few hours to a few days. Some people were very pleased with the wheelchair that was lent by the resort, but just in case you have problems, it's good to have some alternatives with you.
If you decide to rent an
ecv, your choices are in the parks (limited quantitiy, and often run out early in the day) or from one of the outside rental companies. The off-site companies will deliver
ecvs to the resort too.
ECVs are also called scooters and are fairly easy to learn to drive. You don't need any proof of disability to rent one (although you do have to be over 18).
If you ask for a power wheelchair, the parks don't rent those at all and the medical equipment companies will tell you they only rent them to people who are already using one (but maybe don't want to travel by plane with it). Power wheelchairs are controlled by a joystick, more expensive, a little harder to learn to drive and they require some additional set-up. (Just mentioning because some people mix them up and I know of someone who thought she could not rent an ecv because she asked for a power wheelchair and they asked her a bunch of questions).