They should be available anywhere that the limited editions pins are normally sold. They might be in a display case and you'll have to ask the CM at the register for the actual pin (that's how most of the pin shops deal with the LE pins these days).
In terms of sell out...I haven't a good sense anymore on what designs will or will not sell out quickly. My guess (and it's only a guess) is that you'll probably be able to find the pin on the 6th. Alas, Teacher's Day is not a "big" holiday/event and the character (Merlin) is not hugely popular, so it doesn't seem as likely that the re-sellers will grab up as many as they can to sell online.