purchase replacement battery

It might actually be possible to get a replacement battery through the Self-Service store operated by a partner of Apple. But it requires a serial number. I checked with the serial number of my 14" M1 and the top case with keyboard and battery was more than $500. I think the trade in value of a returned top case was maybe $80. For that price I'd recommend going for Apple's battery replacement service, especially since they'll make it right if there's a mistake made that damages the computer.

https://selfservicerepair.com/home

But the OP can maybe have a look to see if it might be worth it. I suspect it's not a good deal, but that's not really for me to decide.

Apple stopped selling batteries as replacement parts years ago. You cannot buy a genuine Apple replacement battery. You must have Apple do the repair. Their total charges are often far less than a private provider charge for the same service, and you ar assured of geting genuine Apple parts.

Also, if your is a Retina Macbook Pro, the battery is not safely user-serviceable. Even if you can survive the toxic and highly flammable solvents required to unglue the battery, the chances of an untrained user causing fatal damage to other internal parts is sky-high.

Use this form to see if Apple will still perform battery service on your computer model:

Mac Repair & Service - Apple Support