Controlled adjustable motor vibrator is so cheap and so easy to make. For instance:
Get a small computer "muffin" fan ($1.00 to $10?). They are small and flat and rated at 5VDC or12VDC. Many sizes to choose from.
They are made of plastic. You can easily break off a few fan blades to make the rotation unbalanced. When it rotates it will vibrate. With a voltage regulator/controller,
you can have the vibration adjusted to the exact jello wobble frequency.