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Do you mean a flexible spacer between handle materials rather than between tang and handle?
I've seen leather spacers. I could also imagine using thin copper and bending it to shape since it's so soft.
hard rubber
Untitled by Bob, on Flickrthanks, will give that a try
You could always use several layers of a cloth that's the right color. Cut the pieces roughly to size, use a credit card to squeegee in epoxy and squeeze it all together. Like home made mycarta, only for a fixed in place liner. I've done it a few times and it works great, the epoxied cloth was just a part of the handle glue-up.