One common theme could be the exploration of their relationship dynamics. It might involve how they interact, whether it's in a friendly way or with some underlying tension.
I'm not sure about specific 'once ler x greed ler fanfic' off the top of my head. You could try searching on popular fanfic websites like Archive of Our Own. They usually have a wide variety of fanfictions in different categories.
There are a few options. ComiXology has a decent collection. Also, Webtoons sometimes has manga available. Just make sure to check the licensing and legality.
It depends on the platform and the author. On Wattpad, many yaoi novels are free as they are often written by amateur authors who are more interested in sharing their stories and getting feedback. But if you go to some official e - book platforms that sell yaoi novels, of course, you need to pay for the books. There are also some donation - based platforms where you can choose to support the author financially if you like the free yaoi novel you've read.
You can try some popular websites like Archive of Our Own (AO3). It has a large collection of various fan - made works including yaoi novels. Another option could be Wattpad, where many independent authors share their stories, and you might be able to find yaoi - themed novels there as well.
I wouldn't recommend looking for free yaoi novels online illegally. It's best to support authors by purchasing or accessing through legal subscription services.