speedometer sensor

I have an 88 vette and need a speedometer sensor (GM pt no 25007308) for a 3.07 ratio rear end. I have tried everywhere with no results - any suggestions where I might look and do I need an exact replacement.

I found this web site that will notify you when the replacement is available.
http://www.1aauto.com/1A/SpeedSensor/Chevrolet/Corvette/1ATRS00020/414180

Hey dickr try this site: http://www.parts123.com/parts123/yb.dll?parta~partsort