Funny you should mention a substitute, Marc, because the Cocktailian article where I found the recipe specifically says "There's no real substitute for Bonal, I'm afraid, but it can be found at good wine stores." The taste is hard to describe but the quinine blends with herbs and makes a really interesting flavor. Here is a comment from some that is helpful:
"For those unfamiliar with Bonal, it's a true Quinquina - quinine is the principal botanical and bittering agent. The base is Mistelle, a fortified unfermented grape must - think Pineau des Charentes or just brandy fortified grape juice. In the home region this is served with a squeze of lemon and on the rocks, maybe some soda. Given the mistelle base it's also nice for stretching out the wood on brown spirits. A simple demonstration is a 1:1 with rye and a dash or two of orange bitters." I really enjoy the Bonal & Rye (2 ounces rye whiskey, 1 ounce Bonal Gentiane-Quina, 1/2 ounce Cointreau, 2 dashes orange bitters, 1 dash Angostura bitters, Orange twist, for garnish) and I'm glad to find more options for drinking it.
Jim, I quite liked the Cortez and should have been rocking the Neil Young, but I actually did that yesterday. The flavor is smokey, chocolately and herbal all at the same time, and the tequila bite shines through. Try it, you may like it.
Next up, In Harvard We Trust! (good cassis is a must for this one!)