I asked my local bike shop to get me a second 400w battery for my 2019 ebike. My bike mfr rep was there that day, we had a great conversation, and a few days later he found a barely used battery (charged twice, 100% capacity, fully vetted) that he sent to the bike shop for me. Essentially brand new. Half price as used (full price new was $800) and $20 shipping rather than the standard $200 shipping cost for new from the mfr battery supply place. So he saved me almost $600. It is exactly the battery that my bike uses, too.
You might want to check with your local bike shop that sold you your bike, and have them contact the manufacturer's rep to see if they can get you a slightly used battery vs a brand new one for your bike. Worth a try, and may save you money in the long run.
Your source really depends on whether your battery housing is proprietary or not - is it molded into the bike frame? Or a hang-on downtube conversion type? A Hailong dolphin or shark pack? Or a big-3?
Another for EM3ev - well recommended as a quality source. Some of the import stuff is a crap-shoot as you really don't know what kind of cell they're using nor how well they're built into the case. It doesn't pay to skimp here.
Also consider if your trips are in legs or a stop-over, like work etc. an extra charger at could provide for a pretty quick top off.