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.