Birkenstock Big Buckles are not sold by Birkenstock to any repair shop or distributor. The only way to get these is off a pair of Birkenstocks. Therefore, the cost is more than a standard buckle. The Big Buckes come in four sizes, 22mm, 25mm, 30mm and 35mm. Make sure you order the correct size. You will need to measure the strap width in millimeters or the cross bar, see red marks in photos.
The Arizona, Gizeh, Madrid and Milano use either the 30mm or the 35mm Big Buckles for the larger straps. The Milano uses either the 22mm or the 25mm Big Buckles for the smaller rear straps. Size will dictate which size Big Buckle you will need.
The 25mm or the 30mm Big Buckle will fit on most of the original Arizona, Gizeh Madrid and Milano sandals. Once again, just measure the straps and order the correct size.