The dealer would certainly be the place to buy the spring. If you bought the extended warranty, it may be covered. Otherwise it may take some work.
I can only speculate but it would seem that the roof haves would have to be separated and the affected side raised until it take the tension off the spring. If that is what is required. I'd be sure to do it inside with some help. Depending on the cost, you might want to do both springs on the side at the same time.
2012 Rockwood A122S
2008 Toyota Tundra Double Cab 4X4
Former owner of a 2002 Coleman Niagara GTE