Is there any grants that will help pay for a stair lift for my Dad who's 81yrs?

As TNtechie states the VA might help if he is a Veteran.
You could also check some of the on line local "garage sales" sometimes you can find equipment there. A friend got rid of a ramp and the person that took the ramp repaired the stairs and deck area where the ramp had been...Win/Win.
Ask at the local Senior Services Center if you can put up a note, often when someone is moving a loved one out of the house into a facility they may get rid of a lift. And long shot you might check with some local "Estate Sale" companies and ask if they come across a house with a stair lift to contact you.

If your father is a vet, be sure to check with the VA.

Try your local Office of Aging. My friend was able to get a ramp thru the County disability dept when he lost a leg to diabetes.

I purchased a stairlift for my husband last year. Did a lot of research and found there is no financial assistance for stairlifts. Also had the same issue for outside stairs and getting a wheelchair ramp. No assistance in paying for these “non essential “ items. Sorry