As an end user, you cannot buy a new genuine Apple battery as a service part. Any offered as such are bogus.
The battery is glued to the upper deck with some seriously stubborn adhesive. The solvents required to remove it are toxic and highly flammable, and can ruin other computer parts if it drip. I cannot in good conscience recomment you go DIY. Too many reports here of computers damaged beyond practical repair by attempts to replace the battery at home.
Have Apple do it. For reports here is seems they not only replace the battery but the upper deck, keyboard, and trackpad, all for less than half of what an independent shop has to charge. Your old working parts get refurbished and you get credit for them.
Because of how difficult the replacement is, It appears Apple does not do batteries in-store any more, but sends the devices to a centralized refurbishment center where the solvents can be used without violating a boatload of OSHA, HAZMAT, and fire codes. Therefore, Apple's mail-in service is very good because you save a day on each end.
Start here: Mac Repair and Service - Apple Support
⚠️NOTE: If the old batery is swollen, you cannot mail or ship it.