I believe that Springfield does produce their guns in runs, like many other manufacturers. The Mil-specs are made in Brazil along with the G.I.'s and the Loaded's, and it seems like Springfield concentrates most of it's time into producing the different varieties of the Loaded series, as they are most likely the top sellers.
However, this is not an uncommon problem. I'm not sure if it's just limited production or the huge popularity of the Mil-Spec line, but they are always hard to find. Just shop around the internet and keep checking you local gun stores, if you are lucky you will find one for a decent price.