Well, first of all, FanFiction.net has been around for a long time and has a huge database of fanfictions, including those about Percy and Thalia's relationship. It's easy to navigate and search. Archive of Our Own, on the other hand, is known for its more inclusive and organized approach. It allows for more complex tagging, which means you can filter by things like the type of relationship (romantic in this case), the genre of the story (like adventure or angst), and even specific plot elements. So between these two platforms, you're likely to find some great Percy x Thalia love fanfiction.
You can try Wattpad as well. Although it has a more diverse range of content, there are likely some Percy x Thalia love fanfictions on there. However, FanFiction.net and AO3 are more specialized for this type of fandom - based fanfiction. They usually have a community of dedicated Percy Jackson fans who write and share stories about Percy and Thalia's potential love story.
FanFiction.net is a great place to start. It has a large collection of fan - made stories, and you can easily search for 'Percy x Thalia' to find relevant fanfiction. Another option is Archive of Our Own (AO3), which also has a wide range of works, often with more detailed tagging systems to help you find exactly what you're looking for in terms of the relationship between Percy and Thalia.