You can buy them on Amazon here!
I also made the hundreds, tens, and ones labels to go on top, and bought magnetic border to separate my place value columns:
If you'd like to download the labels that I made to use on your whiteboard too, I would love to share them with you! Please note, that only the 3 labels are included in this download. You need to buy the manipulatives separately. Click on the picture below to snag the labels: