Hi Shirley,
It depends on exactly which part of the power button you need; there's the electrical part that connects the switch to the rest of the phone, and then there's the mechanical part, which is the button you actually push with your finger. If the button isn't functioning it could be either part, but usually it's the electrical circuit, but if it's physically damaged then you need the mechanical part.
Here's a guide showing you how to replace the electrical part.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-note-9-843/pulsador-lateral-de-encendido-7298|Manuales / Samsung Galaxy Note 9 / Ignition side push button|new_window=true]
You can find replacement parts at many online sites; I usually start with Amazon, eBay and AliExpress, but here's one of the first ones that came up on my cursory search for "SAMSUNG GALAXY NOTE 9 power button replacement".
[link|https://www.injuredgadgets.com/samsung/galaxy-note/galaxy-note-9/power-button-flex-for-samsung-galaxy-note-9-n960/|Replacement Power Button Flex Cable Replacement for Samsung Galaxy Note 9 - Injured Gadgets|new_window=true]
If, on the other hand, you actually need the button mounted in the case, that takes a different guide.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-note-9-843/tecla-de-encendido-7301|Manuales / Samsung Galaxy Note 9 / Power key|new_window=true]
Of course, the button comes in different colors, so you'll need to order the one that matches your phone. This seller on Amazon is offering all of the available colors.
[link|https://www.amazon.com/Button-Replacement-Repair-Samsung-Galaxy/dp/B09WDQDYPD?th=1|Amazon.com: Side Power On Off Volume Button Key Replacement Repair Part for Samsung Galaxy Note 9 (Purple) : Cell Phones & Accessories|new_window=true]
Either way, that should get you going!