I pretty much limit myself to what I can find at the generic grocery
stores in the area, and I haven't seen dark chocolate mint chips.
Since you mention chunks as an option, you can make your own
by melting dark chocolate and stirring in mint. You might want to
add a little shortening to make the final product softer. Be careful
that you don't get any water in the chocolate when you melt it,
and don't let it get much warmer than what is necessary to melt it.

I have a friend at work who chops chocolate bars up with a knife
and makes wonderful cookies from them.

Good luck!

Michael