An authorized Apple Service provider, an independent repairer & may be
associated with an independent authorized reseller, could likely replace a
battery in an older portable Mac tablet. Usually after 4 or 5 years the Apple
Store may not even have inventory for ones no longer covered under an
AppleCare extended plan.
My older Apple portables no longer could get new batteries from Apple
so I found suitable ones from newertech via macsales.com OWC site.
For other upgrade or replacement parts see: MY UPGRADES - OWC
Note the OWC site has good videos for a guide in replacing parts.
Their pages also have calibration& other instructions to prepare their
batteries for best service; and those also are helpful to perform if ever
one needed to return a battery to OWC under their good warranty.
Examples: macbook/air batteries for 11-inch & 13-inch, by build year:
https://eshop.macsales.com/search/macbook%20air%20batteries
Third-party sources of replacement batteries should be able to supply one
if the product is of better quality; the least costly likely has lowest quality.
An iFixit.com repair guide can show how to replace a battery in MacBook
models (various) and their site shows some replacement parts as well.
• MacBook/Air Repair Guide - iFixit.com:
https://www.ifixit.com/Device/MacBook_Air
There are two categories of MacBook/Air portable based on display size;
& other sub-categories based on build model series/year & specification.
• Find an Apple Authorized Service Provider
Sorry the experience you've had with an Apple Store(?) or authorized retail
outlet was not good. They should have considered you may have sought
to buy a new computer some day ~ & after this, you'd look elsewhere.
Good luck & happy trails! 🙂