Does any manufacturer make the little rings that slip over the truck bolster or kingpin and give a bit of extra separation between the car body and truck?
How much thickness do you need? What I have done in the past is to cut the tabs off of these motor brush contacts. I ended up with a stash of these left over after installing DCC decoders. I think they are about .005 thick.
In the past I've used the Kadee HO scale Insulating Fiber Washers. https://www.kadee.com/fasteners-c-274_334_281/208-red-insulating-fiber-washers-015in-thick-p-316.htm https://www.kadee.com/fasteners-c-2...nsulating-fiber-washers-010in-thick-p-317.htm
Or you can use those fake credit cards they send you in the mail to get you to apply or used gift cards, etc.
You can buy small washers from places like: https://www.microfasteners.com/home.php?cat=646 I replace the plastic kingpins on my rolling stock with 2-56 screws so any #2 washer will fit
Mike, I have a bunch from Micro Trains bulk packs. PM me with how many you need an the're yours. Charlie