How/Where can I find a used or new rear differintial and axle for 1996 chevy Z-71 truck?

I have a friend who has a 1996 chevrolet Z-71 truck. I'm pretty sure its a 5.3 liter V-8, extended cab, short bed, automatic transmission. The engine is new ( replaced). He wants to sale it, I would like to buy it for an extra vehicle. The problem is that the rear differential is shot/busted. He told me that it has had to be rebuilt several times over the years. He suspects that thier is some other reason that it continuously has to have rear diff replacements. He suggested "replacing everthing between the rear tires" guess that means axle, housing, mounts etc etc. I live in Midland Tx and need to locate the part or parts and price them. Used would be nice, or salvaged. New is an option too but I'de like to keep cost down since truck will be a hobby/toy and not primary vehicle. If I look on line I find parts that are several states away. The shipping will probably cost more than the parts. I have a SUV and flat bed trailer to go pickup parts if I locate a good deal. I'm just wondering if any of Y'all know of a good way to advertise my interest in buying parts? Maybe somebody knows a good website to search for parts? maybe somebody knows someone with this part or similar vehicle that could be bought for salvage? Thnx in advance!