My favourite way to enjoy Chartreuse. Especially good when made with V.E.P. Chartreuse. Use equal parts of each liqueur or use more of the stronger Green...
After giving up on finding some where I am, I have been getting Pagès Verveine du Velay Verte and Jaune. Not quite the same or as good, but available and less expensive than the real stuff.
I suspect availability varies greatly according to geography. Both the green and yellow still seem available at some level in Europe. VEP is harder to find and if you do, even pricier than it used to be.