After unsuccessfully trawling the local hardware or auto parts stores for anything that was large enough and thin enough for the task, I turned to my box of aircraft hardware and pulled out a bunch of AN960-1016L washers.

... which I then snipped...

... and slid in-between the spoke and the felloe.

As you can see, they are an absolutely perfect fit in both inner and outer diameter and the thickness (1/32") was spot on for my application (if you need to fill more gap, washer AN960-1016 (drop the 'L') is thicker, at 1/16")*
Many thanks to MTFCA member Ron Horton for the exceptionally generous loan of his spoke jack.
* this all said, I do not condone shimming the spokes on your car. I don't know what I'm doing. It is both stupid and dangerous. Buy new wheels.