I've done some work on the seat of an engine head, and now i can't find any shim that can i use for setting the valve clearances.
The clearance is set by a 11.5mm in diameter diameter shim that is placed just under the bucket, between the bucket and the valve stem.
The minimum shim that i can find is 2.00mm. I'd suppose that it's difficult to find shim under this value because this shims seem case hardened, and under 2.00mm the core can be too brittle, and can split in two under the load of the valve stem (6mm stem loading something like 60/70kg at max rpm, spring load + inertia).
I need some 0.95 /1.00mm shims, so i must find a way to machine them by my own.
I know that the tip of the valve(Plug Valves) is hardened at least at 53 hrc.
I've think about grinding the shims and get them nitrided, so that the core will be untouched.
Someone have a suggestion for me?

You should take a look at Precision Brand, they as well as others have 1.00 mm feeler gage stock.  You could also look at hardened drill rod, key stock, or even flat stock to get your required 11.5mm thickness.
The majority of feeler gage stock is through hardened.

Measure the surface hardness of a factory shim.  Then gently grind it down to your required thickness.  Measure it again.  If still hard, have it checked for grinding checks via liquid fluorescent mag. particle (Magnaglo).  If the ground surface is softer, nitriding should be able to restore it.

A good nitrided case can be as thin as ~.005", so a .080" shim would be no problem.  Any "white layer" should be removed by lapping, not grinding.  You'd probably need at least a steel like 4140 to get a good hard case.  But there is another way.

Get a piece of pre-hardened S7 tool steel drill rod.  You'll have to grind it to size, but if you do it gently you probably won't crack/heat check it.

S7 is a VERY tough tool steel, even at ~Rc58.  Your shim stresses are probably pure compression, with little or no bending.

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