I got it from here https://hindleys.com/index.php/ Its 1.5mm black-white-black engraving plastic, 600x300 sheet, £14 delivered.

First time i've used it and came out perfect, used a 30deg engraving tool, 0.2mm tip, 24000rpm, 900mm/min, air blast only and used an Aspire V-Carve path. Outer edges were cut with a 2mm single flute carbide bit with 6mm x 0.6mm tabs.