Some big box stores like Wal-Mart are now offering propane sales near their outdoor garden department.
They sell both new tanks and are able to refill those that are empty. The propane is kept in a secure fenced area, so you will need to get a staff member to give you access to the propane.
As a word of caution, the propane refill service is self-serve and I am not sure if there is anyone on staff that would help you out if you needed this service. If you aren’t feeling super confident about refilling your own tank, I would bring a friend that has done it before.