Sourcing a new battery from reliable venders

It is likely going to be very difficult to find a true OEM replacement, since Garmin never really intended for consumers to replace the batteries in these watches. I did a replacement on my old Fenix 5X a little while back, and I bought a couple of "no name" batteries from Amazon and eBay and tested them. From what I have experienced, the one I chose works great. If you're ok with the slight risk that the battery is not as high of quality as the original, I'd recommend just going ahead with whichever you find that seems most reliable.

If you do find one that works well, please let me know, because I now wear a Fenix 6x, and I am thinking about replacing the battery.

(edit) also be careful to try and find a true LiHV cell (Lithium Ion High Voltage). OEM Garmin batteries have a higher max charging voltage than standard Liion cells so the watch will slowly damage standard Liion cells if you use one as a replacement and the watch's battery life estimation will be off.