I remember there always being menthol crystals in the medicine cabinet since I was little so they've been around for ages. They are used regularly in salves, medicated creams, throat lozenges, cough medicine, oral & throat sprays, mouthwash, toothpaste etc etc as well as for that good ole towel-over-the-head steam inhalant for colds. I've been using menthol crystals in my eliquids for around 9 months now and like it pretty strong and haven't had any horrible side effects. Worst thing I've had if I've overdone the strength is a temporary sore throat, or a feeling akin to inhaling deeply on a very cold day after eating extra strong mints!
There are plenty of herbal, cosmetic or pharmaceutical web sites where you can buy menthol crystals in larger quantities. The size of the crystals will vary (last one I bought came as 4x 25g crystals which I had to crush up) Just make sure that the crystals are good quality (should be 98 to 99% menthol) and they are packaged in a labelled jar, bottle or tub. I never trust ones which are sold in plastic bags as you never know where they've come from