In my area, virtually all the gas station convenience stores have the 20lb propane Blue Rhino exchange tanks. I have not seen an exchange that carries any other size, either 30lb or the little 1 lb stove bottles.
It is getting difficult to find places to fill propane tanks. And "topping off" can get expensive - all I've ever seen around me is "fill by weight", with a set price. If your tank is half full, you get charged for a full filling.
As another posted, call a local campground - if they don't have a fill station, they will know where to go.
Sort of related - I went out yesterday to check on our TT (camping this weekend). One 30lb tank was maybe a 1/4 full, no problem! says I, as I always have at least one full tank. In the process of remounting the tank, I jostled the matching full tank (TT has two tanks on the tongue). Felt odd. Took tank off - surprise! It was completely empty. I have always turned both tanks on, with the crossover valve thing between them. I guess we used a decent amount of propane last year without me realizing it. I think I am going to start only having one tank on at a time now, so I know how much we are using!