|Someone is selling a list of available three, four, and five letter .io domains that are available for $20.|
This - https://gist.github.com/4111298 - generates a list of available .io domains of any length (user specified). And, its free.
(I'm away from my computer, so it isn't tested, but I ran it in repl.it).
Edit - this is the seller - http://news.ycombinator.com/item?id=4804627