We usually buy ours at Trader Joes, and they definitely have the best selection. We also see it at various supermarkets, and at health food chains like Whole Foods and Wild Oats.
Some of the brands that we buy are:
Scharffen Berger (exotic)
Valhrona (my favorite!)
Unique Origin
Lindt
It is the Lindt brand that we see most often in general supermarkets, it's quite good. If you've never had chocolate that was milk-free before, then you are in for a big surprise, because you've never really tasted chocolate before. It will seem very strong, because you've only tasted very diluted chocolate before. Well, it is quite strong, which is why it is so good. After all, if you don't like the taste of chocolate you can just eat a Hershey's bar.
Really good chocolate will list on the label what percentage of cocoa it contains. I think that may be a legal requirement in the European Union. Anything over 50% is good, 70% is like going to heaven without having to die. Anything over 75% would probably result in arrest and prosecution for having too much fun.
I'd be very interested to hear what you think after trying real dark chocolate for the first time.
Steve