It’s a braided, waxed polyester thread. I tried a twisted poly thread (unwaxed, so I had to wax it myself) and some waxed linen thread, and both were significantly tougher to thread through my needles and to sew with. I kept breaking the linen thread when I was pulling it tight, but the tiger thread was really strong and didn’t break at all on any of my projects. I think the actual name brand is “Ritza 25” but it has a tiger on the label and I see most people referring to it as “tiger thread” on the guides and videos I’ve seen. It sounds like it used to be hard to find, but I just did a search for “tiger thread” and found Rocky Mountain Leather Supply. They have it and they do free shipping the US, so I jumped right on it. I just got black, but I’m definitely going to pick up at least a few more colors, too.
This guy’s site and videos were extremely helpful, and if you don’t mind a whole heck of a lot of depth and detail, he’s absolutely great: http://www.ianatkinson.net/leather/leatherguide.htm