are you aware that it would be very difficult to build one on your own?
optical mice use a special sensor, sort of a little camera that is in a windowed IC, to basically look at the ground below it. I doubt you would be able to purchase one of these IC's on its own, and even if you could, i doubt it would be cheap or easy to interface with.
conversely, you can buy cheap no-brand optical mice for under $5 online if you look hard enough... if you're planning to use this for some sort of project, rather than just to try to make your own mouse, then you could very easily just tear a cheap one apart and do with it as you please. interfacing with the PS/2 bus isn't that hard.