front driveshaft help please

i'm looking at buying a '89' gmc jimmy and it has a bad slip spline yoke on the front drive shaft. where do i go to find one. i've checked all the site sponsers. mabye i'm calling this part by the wrong name. this must be a common problem because i looked at a "86" yesterday with the same problem, my best guess is that this is also found on half ton pickups