If there is a flying J on your route it will, Pilot, Loves and TA might, In some areas hardware stores or general gas stations (you knew that last one) U-Haul often has Propane, and Many campgrounds, but you tend to pay more at a Campground.
I like to look for wholesale places or other places that have "Tank farms" (many big tanks) Usually these places have better prices BUT NOT ALWAYS.
Where I am not the local tank farm place is the most expensive.
Where I usually buy propane in the summer the least expensive
And where I got it this last spring, Well not the cheapest, but close enough to it that the cost of gas to get to the cheapest would have negated the savings..... Plus I wished to visit that specific farm again (It is the farm I grew up on). That was nice. I'll have to go back there if the path ever crosses again.