These are what I use when doing these exercises. They usually run somewhere in the middle of the price/quality range, and are often sold in sets of different line weights - remember that for the Drawabox lessons, we only really use the 0.5s, so try and find sets that sell only one size.

Alternatively, if at all possible, going to an art supply store and buying the pens in person is often better because they'll generally sell them individually and allow you to test them out before you buy (to weed out any duds).