Like the Staedtlers, these also come in a set of multiple weights - the ones we use are F. One useful thing in these sets however (if you can't find the pens individually) is that some of the sets come with a brush pen (the B size). These can be helpful in filling out big black areas.

Still, I'd recommend buying these in person if you can, at a proper art supply store. They'll generally let you buy them individually, and also test them out beforehand to weed out any duds.