What he said. It'll be much easier, and cheaper, for you to convert to 1x10. Simply drop the FD and shifter and you're pretty much there. Go a coupl'a steps further and install a chain guide, and whatever size chain ring you wanna push and you're done.

If you go to 1x9 you're gonna need a chain, cassette, 9-speed shifter, and a 9-speed RD.