What do you want to use the diff for though? As it is only really of benefit if you are going racing.. On a street car I believe you wouldn't get value for money.
If you don't want to buy new, you can get the factory R160 plate one from an RA, although I think they are pretty rare and if memory serves me right, only come in 3.9 ratio. As Loren has mentioned for the R180 factory plate LSD, isn't as aggressive as aftermarket, but still better than sh*tty viscous and I would expect the R160 one would be the same.
Otherwise, try calling workshops that work on rally/race cars and see if you can find someone with one they are willing to sell.