I've actually had really good luck getting tickets, at a good price, with only between like 1-6 days out. I would still try and schedule out a bit in advance because waiting until the last minute can certainly backfire. In general, I find that scheduling right under the 3 week mark is the magic number. Also, this makes me sound old, but I also use a travel agent (yes, they still exist) when I can't find what I need/want.