Diameter is 31mm
You have several options:
Buy them at Maserati for 17€ each. The problem is only, you would have to buy a large set or go there each time for each valve. It's hard to know beforehand exactly which thickness you need since also the size of used shims is almost never what is written on it due to wear. The good thing on Maserati OEM shims is that they have smaller steps like 2.90mm > 2.875mm > 2.85mm etc. But in reality it's only useful if you have a tool that can measure in these steps and I rather tend to give them a 0.025mm of extra clearance.
Then there is one seller on ebay where you pay 7€ for each shim but you need to return your old shim so in the end paid a lot, but still don't have a single shim at home.
Then there are several brands out there that produce these shims. The cheapest I found were from Metelli. The sizes you will need will be mostly from 3.05mm downwards to around 2.75mm
Most are between 2.95 and 2.85
The Metelli numbers I have at hand are:
3.05 - 03-0200
3.00 - 03-0199
2.95 - 03-0309
2.90 - 03-0308
2.85 - 03-0307
2.80 - 03-0306
2.75 - 03-0305
2.70 - 03-0304
2.65 - 03-0303
2.60 - 03-0302
Last year I could find them for around 2-4€ each at Motointegator:
https://www.ebay.de/itm/Einstellsch...LLI-03-0308-/151830385281?hash=item2359cbd281
