Speaker for crystal radio rather than sensitive piezoelectric speaker Crystal radio
You might try looking in old Army surplus stores. Old tube type radios used a 2000 ohm headphone. This is the type of headphone that was used on crystal radios in the past century.

The impedance is 2000 ohms, and the resistance is a bit lower. Take an ohmmeter along with you shopping so you can measure the headphone. Anything measuring from 500 to 2000 ohms could work.

You can also search online for 2000 ohm headphone.