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!