I gotta say my search for Menthol Crystals is most memorable. Never had I gotten such strange looks from pharmacy staff. Heck one of them thought I was looking for something else.
You can easily find them at beekeeping supply companies. They use them to treat bees for a particular type of mite. Try Walter Kelley company, I think their website is kelleybees.com They are in Clarkson, Kentucky.