Ive been using my 9.3x57 die to neck up 8mm brass.
Id like to expand it larger, then use the 9.3 die to neck it back down to fit the chamber shoulder snug because Im still getting backed out primers
Questions are, what expander should I buy? Brand and size to buy(cheap but good), a link would be cool.

What is the exact procedure or is a expander a hollowed out die that just holds a expander and rod?
Ive read about doing this but never realy paid attention.

when you expand over the size of the FLS die you have you set the ecpander stem down far enough that it is the only part of the die the case engages with. RCBS, Lyman and the like are a simple fix for this task.
Depends what dies you have, but you could just get a larger expander button for your 9.3x57 stem.
I have sucessfully taken 5.56x57 up to 30 cal by using incremental sized expander buttons on the 7x57 dies. I over expand to a polished 30 cal button and partial neck size to get the fit I want. My reason for doing this was to get the extra thick neck of the 5.56 brass so I could get a fitted neck for a special project.
If you change neck size much it pays to anneal before and after the operation.

Von Gruff.

So just use the die I have and crank the expander stem way down so the case doesnt go up in the die. Then just put on a larger expander button. gotcha.

Wonder if a .375 button is big enough to make a new shoulder or need larger for a 9.3x57 ?

375 should work just fine.

What you want to do is take the entire neck up to 375, then gradually re-size back to 366 until you can close the bolt with quite a bit of feel, moreso than with an AI type chamber. It should be hard to close the bolt, not impossible, but very firm as you turn the bolt handle down. This will stop any stretching and backed out primers.