Wheelchairs are considered medical equipment and are covered by medical insurance. You need a doctor's script to get one.
There are many durable medical equipment suppliers out there who can provide one. Also, you can find pre-owned chairs in local classifieds. They can also be rented and after a certain amount of time, you would own the chair.
Google Durable Medical Equiptment and you will see many providers where you can purchase a chair.