Have you tried running a diagnostic test to troubleshoot the vibrator? You can run a diagnostics test on your HTC by opening the phone dialer and typing in *#*#3424#*#*
By typing *#*#3424#*#* the screen should display a menu which allows you to test different functions of your phone such as the vibrator motor, speakers, flash light, touch function, back light and others.
The HTC one M9 vibrator motor is soldered directly to the motherboard while the HTC one M8 vibrator motor has a small connector that plugs into a connection on the motherboard. So if you are having issues with the vibrator motor on your M9 then you will likely have to do a little soldering work to fix the issue. You may want to remove the back cover on the phone and see if maybe the wires for the vibrator motor have broken loose from the motherboard. If so you may just need to resolder the two wires back in place on the motherboard to fix your issue.
If the vibrator motor is properly soldered to the motherboard but is not functioning properly then you could use the vibrator from the M8 and cut the small connector off the end of the two wires and solder the two wires directly to the M9 motherboard. I too have looked for a vibrator specifically for the M9 and have only seen this site which sells the M8 vibrator which is listed for the M9 also. Here the link to that site: https://www.gultek.eu/One-M9/vibrator.ht...
Hope this is helpful to you. Let me know if you have any more questions and best of luck.