Is there any financial assistance with the purchase of a wheelchair?

Some Senior Centers have a loan closet. You wouldn't own the wheelchair but could borrow it. Have your father test it(them) to make sure he finds one that best suits him.

If his walking ability has diminished to the point that it appears he'll need a wheelchair indefinitely, make sure the wheelchair has removable arms and leg rests. The removable arms are important because if he reaches a point of not being able to stand safely to transfer, you can buy or make a slide board which can be used for him to "scooch" over from the wheelchair onto another seat, or perhaps even a bed (depending on its height).

Is there a reason his doctor hasn't scripted for a wheelchair?

You can receive 1 wheelchair every 5 years! Make sure it's the one you want and it fits your loved one. I learned the hard way.

Medicare typically pays for one wheel chair. With the help of his doctor, the proper type should be decided upon. Having said that, just about EVERY second hand place has a massive supply of durable medical equipment. Medicare pays and when the item is no longer needed, people are desperate to get it out of their homes as these items take up so much space.
A church near me has a medical equipment ministry that accepts such items and gives them out on loans to whomever needs the item. Nursing homes might be a good source as well. Good luck