Any automotive supply store should have gasket material in whatever thickness you need. They would have different types of gasket material from paper to cork and have seen a synthetic type of material that does look like rubber. They are sold in various sizes of sheets and sold separately. You could use your existing gasket as a template to make a new one. If the old gasket is far gone, use the spotlight mounting bracket to make as a template for the gasket.
Hope this helps.