I have just purchased 4 of these from Apple4less.com. based in Ireland. But you need to purchase the screws separately!!
Apple Part No. for the tray/sledge/caddy/bracket/carrier is: 922-7728.
Apple4less part code for the screws is: HDD-SCR-MP-2
I paid just over €14 for each tray, and an extra €7.50 for each set of 4 screws.
Note that earlier versions of MacPro (series 1-4) require a different, shorter tray than later versions. The quoted part number is for the shorter version. My MacPro is a 3.1