This is a 1-meter long USB cable, one end of which has an "A" connector that works with most laptop and desktop computers, the other end of which has a "micro B" connector that works with all current Altus Metrum products.
We buy these from Monoprice where the item number is 4867. We include one of these cables in each of our Starter Packs.
Note that this cable is not compatible with TeleMetrum version 1 boards, which used the older-style "mini" USB connectors. It's not hard to find mini cables, but we don't stock any in the store at this time.