What kind of toy keyboard are you making?

You can also consider Tone.js which is a synthesizer. Also, I like the sound of https://github.com/PencilCode/musical.js

Those two above will allow you to make a simple website toy piano.

If you are making a command line app, you can record the samples from GarageBand if you have a Mac.

There are other cool command line options. If you're interested, let me know.