Fanfiction.net is also a great place to look. There are many different types of fanfictions there. Just type in 'Percy and Thalia love' in the search box, and it will show you relevant stories. However, the quality may vary, so you might need to do some browsing to find the really good ones.
Another option is Wattpad. Some authors post their Percy and Thalia love fanfictions there. You can explore different genres within the Percy and Thalia relationship, like romantic adventures or more emotional, slow - burn romances.