VoidMist or ThePerfumersApprentice (TPA). Both places sell what is called liquid menthol concentrate, which is menthol crystals dissolved in PG or PGA (pure grain alcohol). Use about 3% (1 ml per 30) to get a clean, pure menthol.
If you want a menthol tobacco, then you also need a tobacco base. I expect voidmist carries several tobacco flavorings, and know that TPA does.
Check out the DIY e-liquid forum area here at ECF, under the Tips and Tricks area. There are several eliquid calculators available for free, which should have some decent menthol recipes.
Good luck!