Shims are way more accurate than necessary. Visit a hobby store or well-stocked hardware store and pick up a couple flat washers to fit. As you retune the carb if you need a bit less thickness, and anvil and a hammer will take care of that, as will a few minutes and a piece of emery paper. Fact is, even cheap flat washers are more accurate than the needles with multiple clip positions which served us old timers well for years.