1988 1500 speedometer speed sensor gear

Hi, I have a 1988 Chevy 1500 pu with a thm 400 transmission. When I got the truck the speedometer didn't work. I pulled out the speed sensor and there wasn't a gear in the tranny (I'm fairly sure there should be). Does anyone know where I can get a gear??? I went to the Chevy dealer and they wanted to sell me the whole shaft. Please help!!! Thanks.

I have a 1988 Chevy Silverado 1500 w/700r4. I had to change it and the one that I bought had the old style mechanically driven speedo instead of the electrical one. I had to have the entire shaft changed. BTW, don't go to the chevy dealer, just go to a good local trans shop, most likely they will just send the trans out anyway~