According to Xoxide's product page, they're made from actual 9mm brass.
I had some spent 9mm casings lying around, so I decided to compare them to the 360 controller's buttons with a dressmaker's tape and sure enough. They're both right at 9 millimeters. The tricky part is cutting them down without crushing them, and then mounting the plastic base from an actual 360 button so it interfaces with the membrane switch correctly.
I've torn down and modified or cleaned these controllers a bunch of times. One time, I even swapped in the thumbsticks from a DualShock 3. Those need some dremeling and other adjustments to get them to fit properly, though.