GM speed sensors on newer cars are usually in the transmission where the speedo gear would be. If the trans has been swapped out and there is no way to install a speed sensor, you might want to seek out a cable driven speedo.

If the trans is a late model unit, check the speed sensor at the back of the trans to make sure everything is hooked up and that there is no broken pieces.

If the problem is definitely in the speedo, these can be sent to your local speedo repair place (most metropolitan areas will have one to three places that specialize).