Online platforms like Amazon or Barnes & Noble's website are good places to start. They usually have a wide range of light novels available. You can search for 'leviathan of covalent light novel' on their search bars. Also, some specialized Japanese or Asian literature websites might carry it as well.
Since I don't have direct knowledge of this light novel, it might be about a leviathan - like being that is connected to covalent in an unexpected way. It could be that covalent represents a form of power, a force that binds things together in the story's universe, and the leviathan is either a creation of this covalent force or is in some way fighting against it. Maybe there are characters who are trying to understand or control the leviathan using covalent - related knowledge.
You could try searching on popular fanfic websites like Archive of Our Own or FanFiction.net.
One option could be to search on dedicated manhwa websites. Some well-known ones are Manganelo or Kissmanga. But be careful of potential ads and the legality of the source.
I'm not entirely sure as I haven't read it specifically. But generally, a light novel often has elements like interesting characters, a unique plot. It might be about a story where 'leviathan' is a central figure, perhaps a creature or a symbol in a fictional world filled with adventures, magic or some sort of complex relationships among the characters.
There are also some websites dedicated to free graphic novels. However, make sure they are legal. For example, Webtoon has a wide range of comics and sometimes graphic novels. You can search for 'Leviathan' there. But always be cautious about the legality and safety of the website.
Well, another place to look could be some anime - related forums. There are often sections dedicated to fanfiction sharing. Members might have written or know of some great Highschool DxD Katerea Leviathan fanfictions. You can also ask for recommendations directly in those forums.
You can start by looking on popular fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions on various topics, including Leviathan Castiel and Dean. Just use the search function and type in relevant keywords. Another option could be Fanfiction.net, although AO3 generally has more diverse and inclusive content.
Since I haven't read it, I can't say for sure who the main characters are. They could be young heroes on a quest, or perhaps scholars researching the leviathan.
You could also check local libraries. Although they might not always have the most up - to - date or niche light novels, it's still possible that they have a copy of 'puyo light novel' in their collection. Another option is to ask in fan communities or groups dedicated to light novels. Members there might be able to point you to the right places to find it.
You can try looking for it in local bookstores that have a section for light novels. Sometimes they carry less - known titles.