If you are on good terms with your LBS you can go and ask if they have any of those rear mudguard chainstay clips. I managed to get a free one from my LBS from their workshop box of spares by asking specifically for that part, and asking nicely, obvs.
If you need some kind of doohickey your answer is almost always on SJS
https://www.sjscycles.co.uk/mudguards/0-thorn-mudguard-spacer-anodized-black-20mm/
https://www.sjscycles.co.uk/mudguards/gilles-berthoud-fixing-bracket-set-for-rear-fender-on-brake-bridge/
https://www.sjscycles.co.uk/mudguards/sks-stainless-steel-rear-mudguard-chainstay-bridge-fitting-clip/
But also go to your LBS and look in their parts box they'll probably have something.