Pretty much, any RV repair shop will sell re-fill propane as do some Home Improvement stores, sometimes boat marinas.
I found I preferred to have my -previous- 20 pound's refilled instead of exchanged. If I have a good tank, I want to keep it. I no longer have 20 pounders at all, only 30, and those can't be exchanged anyway.
If in doubt, check with your campground host, campground park ranger, or campground store or local bait and tackle shop. There is always someone who will be able to tell you where you can get fill up's opposed to exchanges.