Just about any liquor store that carries kegs of beer.
tell that that you will want to buy all you kegs from them and open a home tap account. My local store fills them for free, as long as I keep buying from them.
I know this thread is old, but don’t forget about welding shops. I live near one that swaps out 5 lb CO2 tanks. You walk in and walk out in under 2 minutes but you do have to pay for it.