It looks like Hot Topic has expanded their pony stock once again, adding both buttons and belt buckles. It seems like they primarily focus on Rainbow Dash over there, and I'm assuming it's doing well if these have popped up.
I originally thought it was a seat belt, but they apparently all look like that! Never was one for belts.
This 20% cooler pants holding device clocks in at $26, and the button is just $3.