I agree with bozo. You need to utilize the search button, because a million others have asked the same question.
But, I'll tell you that on my previous bike, I installed this Speedo, wich happens to have a cool little odometer on it. Worked great for me, plus it was very cheap.
...i bought one of these today on the bay, the seller says it's brand new and the genuine article and not a chrome-plated plastic knock-off, drive cable and mounting hardware included.
...clik the pic for more details if you're interested.