How to find loudspeaker

Sorry but that's TS, there is no way a small 30mm speaker with be loud enough.

If you use a small impedance matching transformer a piezo transducer might be loud enough although I doubt it'll produce a very nice ding dong sound.

Why does the speaker need to be so small?

Use a larger case so you can use a larger speaker.

Why are you trying to make an electronic doorbell with a tiny earphones speaker? Electric/mechanical doorbells are fine and are cheap.

I had my home lengthened then my American electric/mechanical doorbell wasn't loud enough. I saw the same one in the store for a very high price and also saw a cheap Chinese one in a plastic case. The American one has a metal case and brass bell tubes hanging. The Chinese one has plastic resonant cavities beside the metal bell tubes that make it much louder and the tones are much smoother.
It works perfectly.