We would typically make these using the 0.8mm thick magnet sheets. We typically carry 2 stock sizes; 50 x 13mm x 0.8mm (96 pieces per sheet) or 30 x 13mm x 0.8mm (160 pieces per sheet).

We do however have knife sizes that can be used in combination to make a size closest to your requirements, namely 13mm, 23mm, 30mm, 50mm, or 80mm.