Is it possible to exchange the original battery with a bigger one?

As far as I know, there isn’t a higher capacity option from my suppliers for iPads but they do make them for iPhones. Apple does charge $99 USD for iPad battery replacement and most independent shops charge that same price, sometimes even more but they can usually get it done a lot quicker and back it with a better warranty. If you want more battery, I’d suggest picking up a portable battery bank like this one from Anker: https://www.amazon.com/Portable-Charger-...

I have had that exact one for almost 4 years now and it’s still going strong. That should be capable of charging your iPad twice before that battery needs a charge.