I'm not 100% sure about a good source for drum pad controllers as components but I know there are hundreds of different types of practice pads available which might be possible to convert into a midi drum pad.
Typically drum pads will either have a piezo element vibration sensor or a pressure sensors (FSR) as the main sensing mechanism. You achieve some really nice results from using a piezo disk as this basically acts like a contact mic, capturing any vibrations on the surface of the drum. You can treat it like an audio signal from which you can then extract the onset and amplitude for your MIDI note. Or you can use the audio to directly drive some interesting synthesis models.
For the pressure sensors there are some issues with hysteresis (a lag in the changing of values) which means they might not be able to catch as many hits as a piezo, but tbh if you are reading from this sensor at a high sampling rate (like the 22.05kHz of the analog inputs on Bela) then you will be okay in most cases.
How you mount these sensors will also make a big difference. As the piezo disk is basically a contact mic you can mount it anywhere on the striking surface and get a pretty good result although you will have a stronger signal closer to the disk. With the FSR you need to think about the mechanism of the drum pad a bit more and perhaps balance the whole striking surface on a smaller piece of material which sits on top of the FSR, translating all the energy that is put into the top surface to the reading on the sensor.
For an implementation using piezo disks check out this post: https://blog.bela.io/making-a-percussion-instrument-with-bela/
and this one: https://blog.bela.io/steamy-interactive-sound-sculpture/
