I am thinking about buying a car and might possibly pay to put in a 12 bolt with it. What all is needed to swap over to a 12 bolt, how much does it cost approximately and where should I go to get one?

if you want a 12bolt, you can get one from Spohn, you need to supply brakes. total, your looking at around 3k

You're not going to find a junkyard 12 bolt that will fit. The only way to get one for a third gen is to buy an aftermarket versions. As mentioned above, expect to pay about $3,000.

I would contact Moser, Strange, and Currie they all make bolt in 12 bolt rears for our cars that work with the factory torque arm suspension. Like the others said expect to pay somewhere around 2500-3000 total by the time you buy the brakes and whatever else you need. Its not too bad to install, you may have to redo your rear brake lines but thats about it everything else bolts right in.

Hmmm thats funny my brother got my 12 bolt out of the junkyard for $500 with the 3.73's and posi unit already in it and it fits like a glove with some minor mods, It was his lucky day though.

Out of a third gen to fit into a third gen? The 12 bolt couldn't be original so he got a good deal if it was a direct fit. I'm saying you're not going to find a 12 bolt in the junk yard out of some other type of car that's going to be a direct swap into a third gen. The 3rd/4th gen uses a torque arm suspension. Every other factory 12 bolt will either be a coil (G/A body) or leaf spring suspension. Adapting any other sort of diff into a third gen means finding some way to attach or eliminate the torque arm.

I found a Ford 9" center section with posi and 4.11 gears out of a Bronco for $75

lol $90. Yeah we had to modify the torque arm to get it to fit.