Another update, a year later. Fifth Season Gardening (north-central Greensboro, on Battleground Ave) is a hydroponics store, and at least 50% of everything they have is fertilizer mixes in liquid and dry form. No simple KNO3, etc. - I asked. Another 25% of their stock is brewing equipment (sounds yummy - I'll probably try it sometime, as it sounds a heck of a lot cheaper than this hobby, and certainly tastes better than anything an aquarium produces. Oh, and they have free samples, too!).
But the main point is that they do swaps, too. Currently, they only keep two 5's and two 20's in stock, and they're both $22. And they're right down the street from me, so it's convenient. Not the best price, especially since my 20 is a nice shiny aluminum jobbie that I'm not going to swap out under any circumstances, but it's a convenient place to fill a 5 when I get caught suddenly needing a refill (which happened this past weekend). Updating original post.