Hello and Welcome to Just Answer.
I just need a little more info to assist better.
Is this the shaft that runs through the engine oil pan that needs replaced?
If it is it does require some special tools to remove the old axle shaft,these are special pullers used to pull out this shaft after the axle and other components have been removed.You will most likely need the oil seal for this shaft that seals this shaft to the oil pan.This can be a very difficult job to perform on this vehicle as components have to be removed in order to replace this.The wheel will have to come off,the axle removed and possibly even the entire hub assembly to gain clearance to install these pullers to remove this shaft and to replace the oil seal.The tool numbers are GM special tool J6125-B and J45104.
This can run up to around 3 hours labor to have this replaced,parts themselves can cost up around $300.I dont know of any specific places you can buy this shaft used.You can try this website below to see if they have one for you but chances are you will probably have to purchase it new.This website below is a compilation of salvage yards in the U.S.Enter your vehicle info and then go through the search list to see if you can find this inner shaft.
This may be best to have replaced by the dealer as it does require some special tools and is easier to do when the vehicle is on a hoist as well.
Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!