I need to space the top cap on an IS headset – allegedly both the original headset (which also didn’t come with a preload washer) and the replacement headset I bought to get a new preload washer are supposed to come with a spacer but tediously don’t. Any thoughts on how to stop the top cap from grinding against the frame?

Sounds like it’s a micro spacer you’re missing, which sit above the compression ring and below the top cap. I would have thought a LBS should have a few knocking about in the workshop if you ask them nicely. Failing that you could try the thinnest headset spacer available, which I think are 1mm, but this might lift the top cap a little too high and compromise the seal.
Edit; If you can’t source one, and have lots of patience, you could make one (or more) from a drinks can! 🙂