The original liqueur formulated by Brother Antoine from the 'elixir' (Chartreuse Elixir Végétal). Green Chartreuse is flavoured with extracts from 132...
As I live in Europe, I fortunately have no problems getting Chartreuse. On my last vacation in Canada, however, I ordered a cocktail with Calendula Montréal Plaza ("Cannes you not", very tasty) in the bar of the Honeyrose Hotel. When I asked, the very nice bartender told me that it was the local alternative to Chartreuse verte and let me try a shot straight. I think it captures the taste of the original pretty well. However, I don't know whether Calendula is also available in the USA or only in Canada.
Chartreuse definitely is a nice addition to any bar! If you see both verte and jaune (or cheaper alternatives), I would skip the yellow one though as imho it just adds a taste of honey and isn't worth the price. Which by now is almost the same in Germany as it is in America, so I might need to look for alternatives pretty soon too 💀