Many of you no doubt have found a local hobby shop that can supply good balsa for you to choose from. The ideal situation would be that this shop or supplier in close by.
National Balsa. Only 2500 miles away by car. Oh, did I mention that they ship?
I have not yet taken advantage of their "custom order" service, but if I needed something specific I certainly would. In the mean time, if I order from the "whatever" pile then by the time I'm done sorting I have everything that I need, and their contest-weight wood is less than wood at the LHS.
If you feel that you absolutely must buy "by hand", look to balsa at craft stores (Michaels, Hobby Lobby, etc.). Most of the people buying balsa don't want the "weak punky wood", so it's common for all the 12-pound stuff to be long gone by the time you get there. Don't expect to find anything below 7 pounds (you won't at your LHS), but there's lots of good stuff at the bottom end of "medium weight".
Given how few people build these days, the economics of hand-selecting wood from a LHS (even a craft store) just doesn't pencil out for you or for the LHS owner. Find a mail-order place that hand-selects wood and knows the difference between A-grain and appalling. Even if you don't factor in the gas money and your time, you'll save money over the prices at your LHS.