I’m assuming you only want to rent one. If so, try Bigelow Chemist on 6th Ave between 8th and 9th (ask for the surgical dept.). They deliver (not just wheelchairs!) and have an excellent homeopathic as well as regular pharmacy. And, if you’re into high-end cosmetics and toiletries, they also excel in that.
I have heard rumors that in other parts of the country, fire departments sometimes have wheelchairs to rent or loan. I was mocked and scorned when I inquired about this in NYC.