Where is the input speed sensor ? And, Can i replace it myself? I know its near the tranny i think.

It's $2000 deep inside the tranny.

Ranger is right, if we are talking about a 4T80, it's behind the channel plate between the chain sprockets.
It reads the teeth on one of the drive sprockets itself.

You have to essentially remove the side of the transmission to get to it.
Most places will charge between 8 and 12 hours for this repair, plus parts.
The ISS itself is not expensive but there are a few gaskets you have to replace and fluid of course.

Honestly I would just rebuild the transmission while you're at it....