1.not that i have had much experience in the field.im not a mechanic.i went for 3.2mm as an experimental guess.it is great for drifting but it draws attention to your car on tight corners with chirps.
ive heard that if you go to little on the shim when it wears down in the first few months,and it will,its kind of like a bedding in process it goes through.so dont go to small.id say if you want it to still be a decent streeter/drifter style diff.take it close to 3.0mm at least.it will feel really tight at first for a few weeks.
you could probably get away with 1.49mm shims on either side and sell or give away the stock shims.
2.buy them from nissan spare parts.as far as i know they are only sold by them.ive been to the uxual repco,supercheap and the like and they all couldnt get them for me.use the part numbers that i provided in the 1st post to get the exact size you want.
3.do it if you are going to get into motorsport.imo it is a waste of time and effort,and it can draw unwanted attention to you on the street,doing it for any other reason.
enjoy it