How can one get a good iPhone battery? Or even getting a new iPhone?

By initiating a battery replacement thru Apple from here:

iPhone Battery Repair & Replacement - Apple Support

The iPhone X is a somewhat dated - but still very capable - phone capable of running both the current and next upcoming version of iOS (16).

They are no longer available from Apple (even as refurbished).

Be very cautious when purchasing a used phone. Read and heed the info here:

The All Too Common SAD Reality of Buying … - Apple Community

iPhone Battery Repair & Replacement - Apple Support

There is no such thing as an iPhone X plus.. There is an XS and an XS Max.. neither of which is sold by Apple as they are ~4 year old models.

If you go to Apple's website and select iPhones, you can see what's available.

iPhone - Apple

Otherwise you can check with your carrier and see what they have for upgrade options.

Keep in mind that if you find an XS Max somewhere, that model was discontinued in 2019, so it's battery will be at least 3 years old, even though to you it'd be a brand new phone