The recommendation below is an advertisement. Most of the links here are part of Amazon's affiliate program (unless otherwise stated), which helps support this website. It's also more than that - it's a hand-picked recommendation of something I've used myself. If you're interested, here is a full list.
Staedtler Pigment Liners
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).