The Perfumers Apprentice also sells menthol liquid. Go to their site, select Shop For Flavors, then select Menthols and Mints, you'll see menthol liquid.
If you have crystals, they can be dissolved in high proof unflavored vodka or pure grain alcohol (everclear). Flavored things (generic whiskey, etc.) are generally not recommended for vaping, due to uncertainties as to just what you would be sucking in as well as various stuff clogging atomizers.
Crushing the crystals helps get them to dissolve faster, or putting the bottle with the crystals and liquid into a warm water bath.