What other paintings has he made? I'm only familiar with Nighthawks.
He has made numerous paintings including People in the Sun and Two Comedians
Oh awesome!  What are his most famous works?
Hotel Lobby and Chop Suey are pretty famous as well as people in the sun. There are many that people find delightful
How much would it cost to buy a print, and how much for a painted reproduction? I can't afford an original.
I can't find that answer but they say it can be anywhere from 50 to 75% cheaper then originals. You can find out more info at artsheaven.com
And prints? Do people sell prints of his work?
It looks like you can customize what paintings you want online and they send a print to your home
What sizes are available for Nighthawks?
I am assumming multiple sizes and they provide frames as well as things to hang. Icanvas is the name of company and use high definition prints to make it most realistic