If you're picking it out with a magnet it's not aluminum and might be coming from somewhere else.
You can find finer increments of shims if needed but I didn't find it necessary. I got as many of mine as close as possible playing shim musical chairs, and then ordered exactly what I needed to get the others in spec. I followed the advice here that .003" is way too tight and aimed for the recommended .005" (.13mm). Given the choice I went with .006" over .004" (one .05mm shim size) and counted .007" as too loose.