Some fandom - related forums or communities might also be a good source. For example, if there is a dedicated forum for the series that Ren and Kyoko are from, fans might share their own fanfictions or recommend good ones they've read.
Another place to check is FanFiction.net. There are many authors who post their works there, and it's possible to find some great fanfictions about Ren and Kyoko's love. Just make sure to filter the results based on your preferences.