Where to get Ultegra R8000 Rear Derailleur bolt

in Workshop
Somehow in between removing my Ultegra R8000 rear derailleur to replace the hanger and then needing to wait to order a new cable I have managed to lose or put somewhere too safe, the bolt that secures the rear derailleur to the hanger.
Given I can’t now go into my local bike shop to ask, does anyone know where online I can purchase a replacement bolt. I have looked online and can’t see anything. The bolt on these latest shimano derailleurs is very different to previous generations ie flat head Allen key head, looks like 10mm diameter and I guess would be around 10-12mm long (no idea of thread pitch)
I am starting to think the only way is to buy a new derailleur which seems crazy.
Any ideas?
Thanks
Chris
Given I can’t now go into my local bike shop to ask, does anyone know where online I can purchase a replacement bolt. I have looked online and can’t see anything. The bolt on these latest shimano derailleurs is very different to previous generations ie flat head Allen key head, looks like 10mm diameter and I guess would be around 10-12mm long (no idea of thread pitch)
I am starting to think the only way is to buy a new derailleur which seems crazy.
Any ideas?
Thanks
Chris
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https://sjscycles.co.uk/gear-spares/shimano-ultegra-rdr8000-bracket-axle-unit-y3e998020/