Crystal earpieces, or earphones, consist of a piezoelectric crystal attached to a diaphragm (see diagram below.) The fluctuating voltage from the crystal radio's output is applied to the crystal causing it to vibrate. The vibration moves the diaphragm which creates sound waves in the air. Usually this is done in an enclosure with a hole in it to capture the sound waves and release them in one direction. The hole usually has a tube stuck to it so that you can further direct those sound waves into your ear canal.
Where to buy crystal earphones/earpieces
Here are two places you can buy crystal earpieces online:
Homemade/DIY crystal earphone/earpiece
It's also possible to make your own crystal earphones using disk-type piezoelectric crystals gotten from a variety of sources:
Piezo speakers from gift cards
The piezo speaker below is one I got from a musical greeting card. Not all sound producing cards have piezo speakers. Some have electromagnetic speakers. It's hard to tell which type is in the card while at the store without tearing it open but it is possible. This page here has instructions for how to check at the store if the card has the right type of speaker without damaging the card.
Piezo buzzer's can be bought at electronics and consumer electronics stores. I bought the one below at Radio Shack. The packaging on it says:
Part number: 2730060
Normally you apply a DC voltage to it and it produces a loud, annoying buzzing sound. In the photo on the right you can see that I cut off a part of a pen and hot glued it over the hole in the piezo buzzer so that it can better direct the sound into my ear when used as a crystal earphone.
Piezo speakers from microwave ovens
If your microwave oven makes a beeping sound when food is ready then it may have a piezo speaker inside. If the microwave oven has a dial-type of timer that makes a bell ringing sound then it doesn't have a piezo speaker.
In the rightmost photo below you can see that I cut a short length of tubing, a part of a pen, and hot glued it over the hole in the speaker. This is to better direct the sound into my ear.
In my case the piezo speaker from the microwave oven had its back exposed. My fear was that some of the sound would escape in this direction when really I wanted it all to go into my ear through the tube glued to the other side. So, as the photos below show, I hot glued a piece of hard plastic over the back.
Video showing how to make a crystal earpiece using a microwave oven speaker
The following is a very complete video about how to make a crystal earphone from a piezo speaker, in this case from a microwave oven. But even if you got the piezo crystal from any of the above sources, this video will still be useful.