You can buy Menthol Crystals from lots of places, most E-liquid places that sell DIY supplies have them and so does Ebay. I make my own drops with them that I add to most juices and make my own pure menthol flavored liquid. Most of the recipes for Menthol drops call for pure grain alcohol which I can't get in VA so I have developed a recipe that is super easy and works well for me.
Fill a bottle 1/3 full of crushed menthol crystals (I use whatever is handy to roll over the plastic bag of crystals to crush them), add VG or PG until the bottle is half full, close the bottle and shake, sit bottle in a bowl of hottest water I can get from the tap (I used to just microwave it for a few seconds but I melted a bottle that way), let it sit there until I remember to come check it (ie next commercial break), shake and check if all crystals have dissolved, if they haven't refill hot water and repeat the sit and soak. When all the crystals are dissolved fill the rest of the bottle with vodka.
Makes good strong menthol drops and takes very little effort or supplies. Also very cheap to make.