According to the CAA, drone flyers must label their drone or model aircraft with registered Operator ID.
There are eBay sellers list physical identity card and printed clear sticky labels. Most of them charges at least £3.99 for a couple of sticky label and £9+ for a physical operator card.
Bear in mind that Operator ID can or may change yearly, repurchasing them is probably not an economically viable solution - especially these cards are not served as an official CAA certification or proof.
The DIY Way
DIY Transparent Operator ID
It’s rather easy to create transparent sticky labels with laser printer & sticky tapes. Here is a guide on Youtube
e.g. My free! DIY Label for DJI Mini 2:
Digital Operator ID
Since you are only required to proof that you are carrying the Operator ID, it’s easy to create a digital one with Apple Wallet or Android equivalent solution.
e.g. My free! digital Operator ID:
Happy flying! ❤️