I actually use all kinds of second hand stuff. But most of the time though, I use leather from old couches.
In this case most (if not all) parts were made from old couch leather (1 a 2 mm). Only the shoulderarmour was made with an old jacket.
Btw, it is indeed sewn, but unlike most people want you to believe, a sewing machine is more than able to sow leather (you just have be a bit more carefull and use a proper leather needle). I remembering sewing my first leather parts with a "denim" needle, it worked, but took some effort. The special leather needles have a cutting edge to cut tiny holes in the leather, which help you sew the stuff together .
I was using leather upwards of 3 and 4 millimeters, and often multiple layers. No way my sewing machine could handle that even with the best needles! How does the thin leather keep its shape? Do you line it with something, or just leave it to fall how it will?
I use several methods to keep it in shape. Sometimes I double up, sometimes I fold edges (which makes it sturdier at the... edges XD) and if I really want to add some strength I back it with plastic (several kinds of thickness). To make it appear that the leather is thicker or bulkier I sometimes use foam (like the edward vest).