I'd try Craigslist as a first attempt for both. Refills seem to depend on your area and where you go. Gas companies (welding suppliers, etc) seem the most expensive. I paid $13 to fill my 5# cylinder.. forget what I paid for my 20#er. But, the least expensive in my area.. by FAR, was a fire extinguisher company. They can also recert tanks that are out of date.
Hop Song Brewing