I usually serge with 4 threads – two needles and two loopers. For basic serging, I use cones of neutral Maxilock serger thread on the loopers and the right needle, and a matching thread in regular spool sewing thread on the left needle. That way, if there’s any peak-through on the seam, especially if you’re crossing another seams and there’s a hump, it won’t look funny, because that’s a place where you can get peak-through of your thread. For really stretchy things, like 4-way lycra knits, I use Wooly Nylon or the Maxilock stretch thread on my loopers. The Wooly/stretch threads are also really good for rolled hems on things like napkins. The thread rolls and fills the edge better than regular serger thread, which is pretty thin and not at all stretchy. Hands down, the best place to buy serger thread is at an online store called Wawak. I’m in an online serger group, and no one has found any place that comes close to their price and variety.