I believe there are two battery types, and your swapping the type being limited for the type that isn’t.

I’ve used batteriesplus, but it’s been a few years since I’ve needed a replacement so I don’t know if they’re in stock.

They’re supposedly offering free battery replacements.

https://support.google.com/pixelphone/answer/16340779?hl=en

If your Pixel 6a is an Impacted Device, you may be eligible for a battery replacement at no charge. You can visit our registration page to find a battery replacement option near you.