The shims are in a pack of assorted small washers. #4 is the size, I believe.
Just be sure whereever you get them, that the O.D. of the washer doesn't hang up inside the slide. Radio Shack doesn't always get their washers from the same supplier, or if they do, their supplier doesn't always make them with the same O.D.
Your best bet is to take a slide and a needle with you to the store(s) when you shop for washers, otherwise, you have to take the time to make the washers small enough to fit.