Where to find isolating shoulder washers for panel adapters

Hi Folks,

I need a source for isolating shoulder/flang washers, the same as the ones attached to these RCA connectors

Yellow RCA/Phono Female F to F Panel Mount Adapter for Wall Plate Chassis Mount

To fit

--9.5mm connector diameter
--10.5mm rack hole diameter

Essentially, I am using a metal 1U rack mount to create a patch panel for Amp and Speaker testing and have a job lot of RCA and Speaker Binding through adapters. The Speakers ones didn't come with isolating washers and are the same size as the standard RCA ones.

Appreciate any links. Thank you