What would be the best forum to request an author to write a story on a particular subject?
There are quite a few request forums, all which can focus on a particular subject/genre/setting. Are you looking to:
Create a game between two writing partners on a specific story idea? Post your idea in the On on One Requests
Create a game with more than two writing partners? Then the Group Forum
Looking to write one specific idea with someone else that has a set ending/may be a short game? Try the One Shot Story
There is also the Non-Forum Request
area if you would like to write your idea of IM, email, PM, or other formats.
And if you are looking for help writing your own story with only yourself as an author, there is the Literary Discussion
area and the World Building
That was a lot of links but I wasn't exactly sure what you might need. Hope something helps!