I've only seen anodized aluminium (blue, red, green) listed at Mammoth:
https://www.mammothelectronics.com/products/ka-15x13-series-aluminum-knurled-knob-15-x-13mm?variant=28796395911 but I live in Europe (France) and shipping costs + VAT kill it...
are there any other shops you're aware of that might have some? Europe would be great...