Anything that's 6 volt will do. I'd suggest a switched one that'll handle 12v as well since you're likely to end up with something 12v at some point.

Generic from the Gnome Depot.
https://www.homedepot.com/p/Vector-6-Volt-12-Volt-Battery-Portable-Car-Battery-Charger-and-Maintainer-BM315V/310386119#product-overview

Deltran, who kinda define the category.

https://www.amazon.com/Battery-Tender-022-0196-Volt/dp/B01L0HTX72

The NOCO unit from atothesquiz looks like a neat unit as well. My only gripe with them is their nifty, but proprietary connector where others use SAE since I use my SAE connector for other things at times (tire pumps, heated gloves) I believe they do have an adapter to get around that, but that becomes one more piece of kit to track.