Monzo seem to use Tag Systems to manufacture the Coral Core cards, and they also offer a sticker form factor, but not the micro one that goes into bPay.
Gemalto (I think they are behind the one in bPay) do offer the tiny ones that can be put into bands and rings.
All these form factors are doubtless at a premium over “plain” cards, but I’m sure the people who wanted this would pay it (would I get a Hot Coral ring? hmm perhaps, maybe if it was carbon and coral, like these glowing ones). The problem at the moment is that Monzo don’t seem to support multiple active cards linked to one account.