Making homemade piezoelectric crystals, testing crystal ignitor from a barbeque lighter, extracting piezoelectric disks from gift cards, buzzers and in general, playing with piezoelectricity.
Piezoelectric crystals can be made from scratch using household products found in grocery stores and maybe even your kitchen: baking soda, cream of tartar, distilled water, ...
You can buy piezoelectric crystals from a variety of places online, from searching for "piezoelectric" on ebay.com or your country's version of ebay, other marketplaces or commercial manufacturers and retailers.
You can also find a variety of types of piezoelectric crystals in all sorts of things that you can take apart, some of which are shown below.
In the photo below you'll find the following:
You might be able to find a piezoelectric crystal in a microwave oven as well. 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. On the second half of my page about crystal earphones I show how to get one of these from a microwave oven.
Below are some webpages where I show examples of taking piezoelectric crystals from things.