The iPad User Guide provides information about adding fonts to your iPad. The Guide is a rich source of frequently overlooked information about your iPad and iPadOS. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
Here is a direct link to the relevant section of the Guide for adding fonts to your iPad:
Install and manage fonts on iPad - Apple Support
Many fonts are licensed - and as such require payment of a fee. Others are royalty free - and can be downloaded and installed without payment for their use. That said, many font-installer Apps may be a paid-App in their own right.
There are also Apps that include a font-installer - one being Adobe Creative Cloud:
https://apps.apple.com/app/adobe-creative-cloud/id852473028
This App allows you to install and use many Adobe and royalty-free fonts without charge; you simply need to have a free Adobe account - and be signed-in to your account to install fonts. The font-installation may need to be occasionally refreshed, but this is minor inconvenience for the benefit gained.
A popular (low-cost) third-party font installer App, often recommended within the Apple Support Community, is AnyFont:
https://apps.apple.com/app/anyfont/id821560738
You should be aware that while installed fonts are usable with many productivity Apps, this feature is by no means universal; Apps themselves must support installed fonts.