I'm assuming you've tried Halfords or similiar?

Yep. They've only got tax disc holders. The parking permit is square, not round. They couldn't think of anywhere else to get them. Looking at it, probably I'd be best going to a garage and seeing if they've got any as promotional items.

I've emailed the company directly, to see if they've got any samples they can let me have as a one-off.

Otherwise, it's sticky tape or something. Not ideal.