asstr.org, best site for any sex story about any fetish.
Best to use google to search it, though, like so:

site:asstr.org yourkeywords (goes by typical standard story codes) (i.e F/b would be older female younger male, etc.)

There's also eunuch.org. It's not ALL theme-related to the site it appears to be (castration, basically, neutering etc.) - a lot of its stories in its archives are a mix-mesh of very different and numerous fetishes.