If you search for 'Piezo Ceramics' you will soon find that there are several (not a huge number) suppliers. Most of these suppliers however require that you send them the specifications of the required parts so they can make you an offer for something that is basically custom made.

Only recently some suppliers offer some specific parts from stock

The German company PI (Physik Instrumente) has a store with some selected discs at reasonable prices. They also have really nice section with technical info on Piezo materials

STEMiNC (Steiner and Martin Inc) Sell parts in small quantities, but the shipping and handling are relatively expensive if you only need one or two pieces.

And of course there is China. There are plenty of manufacturers there, and thank to AliExpress these are also approachable to us (those who do not speak or read Chinese ...)  Here the choice is almost overwhelming, but it's not always clear what is available in small quantities or what the specs are for a specific item. In the end I opted for SOARPiezo. :

Ultrasonic Piezoelectric Ceramic Disc PZT 40x4mm-PZT5

A This disc has a 'thickness frequency" of 512 kHz which is a frequency that is commonly used for short / medium range echosounders. Depending on circumstances, ranges of 50 to 100 m should be possible.
These discs are €5,20 each, and with a €7,- shipping that is not too bad. So I bought 4, and they arrived in less than 2 weeks:

Nice touch: they marked the '+' side. Not relevant if it is used  as a single transducer echosounder, but very nice to know if you want to build an array where the phase of the individual transducers must match.