Where can I find a reliable battery replacement for my MacBook Air 2020 (A2719)?

I would recommend shopping at OWC’s website to find your battery. their batteries are the ones that many here (including myself) will recommend here. and there are self install videos on their site so you can determine if it is an operation you are capable of tackling, or if you will want to farm it out.

but I really should have added, that Mac is recent enough that you should be able to source an Apple OEM battery, and have an Apple Store, or an AASP replace the battery. IMHO, that would be your best bet.