Steerer tube shims for quill stem?
Threadless quill stem adaptors do commonly come with a shim, so that they can be used with either size steerer, e.g. 
http://harriscyclery.net/product/velowerx-universal-stem-adapter-quill-to-threadless-5423.htm

I've never seen such a shim sold separately, but a search shows up https://www.amazon.co.uk/Alloy-Aluminium-Quill-Adapter-Quillstem/dp/B00K7ECRLO, which is currently unavailable, but was sold at some time, so you might find one.