Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfictions, and you can easily search for Mavis x Peter Parker stories using the search bar.
Fanfiction.net might also have some. Although it has a broader range of content, you can still look for Mavis x Peter Parker fanfiction by using the relevant search terms. Just be aware that the quality might vary more compared to AO3.
The best chapter pointer here is Chapter 43, "How can sitting in business class be this uncomfortable?". It opens with The moment He Yun willingly sat next to Jiang Li and even took off his... and closes on Just as she turned her gaze away, the other party huffed and withdrew her....
Chapter 68 is the strongest chapter-level answer because it starts from Jiang Li carried the noodles straight out of the kitchen and came to the... and follows through on But didn't Uncle Zhao say he was still sick?.
The best chapter pointer here is Chapter 61, "Young Master Before vs After Going to the Countryside". It opens with Hahahaha, oh my god, Jiang Li's mouth is even more venomous. and closes on A cold shower is pretty normal, right?.
The best chapter pointer here is Chapter 75, "Master, I didn't mean to bump into you!". It opens with The ordinary people in the livestream were all stirred up by this scene. and closes on But, being a half-baked practitioner is just fine..
Chapter 74 is the strongest chapter-level answer because it starts from Having seen Jiang Li's ID before, he knew that this person named Shiguang... and follows through on The man's phoenix eyes narrowed slightly, he looked at the girl sitting....
If you want the clearest chapter reference, go to Chapter 71. Early on, Shu Yan curled her lips with a hint of jealousy, "When can it be my turn...; later, the chapter leans on Who in this circle didn't know that Jiang Li had a thing for the....
A practical answer is Chapter 20, because it turns the idea into a concrete scene. The chapter opens with Several people turned their heads upon hearing the sound and saw three..., adds pressure through Miss Su, please clarify the situation and do not slander me unnecessarily., and ends on So, he.
Chapter 50 is the strongest chapter-level answer because it starts from [666, if good scores determine rankings, is Flash Change's production team... and follows through on What kind of heavenly being had such a sour temper and liked dancing....
A practical answer is Chapter 40, because it turns the idea into a concrete scene. The chapter opens with [Jiang Li claims she copied this herself?, adds pressure through Suddenly, someone typed in the live stream chat— "Oh my god, everyone,..., and ends on The handsome guys on this show are so.