First of all, it helps to know what software you have at the moment.
I have heard of (but never used) a nifty little editor called GXSCC that lets you create pure 8 or 16-bit audio. Im pretty sure it's more used to 8-bit though.
If you want to go a bit more technical about it, then try Logic Pro for Mac OS, and use the Bitcrusher plugin (that is if you have it), the PWM Start synth on ES2 and the Sync Start synth on ES2.
Or, you could just get me to do it. I am writing a varied soundtrack for the project I'm working on consisting of over 40 tracks, and wouldn't mind making another