Yes, there are quite a few. You can start by looking on popular fanfiction websites. Some might focus on their relationship dynamics in the context of the original story's world, exploring how their different personalities interact.
One common theme is friendship development. Given their different backgrounds and personalities, stories often explore how they become friends. For example, Denji's more straightforward and sometimes naive nature might contrast with Kobeni's more timid and cautious self, and the fanfic could show how they bridge that gap.
Another option is Tumblr. There are often fanfic writers who post their works there, and you might be able to find some Himeno x Denji fanfiction by searching relevant tags or following the right blogs.
Sure. You can start by looking on fanfiction.net. There are often a variety of fanfics featuring this pairing. Some might focus on their adventures together in the unique world they are from, exploring how their different personalities interact and complement each other during battles or in day - to - day situations.