You can start by looking on fanfiction.net. It has a large collection of Star Wars fanfics, including many about Ventress. Archive of Our Own (AO3) is also a great place. There, you can find some really well - written and unique takes on Ventress - centric stories.
The best place to find 'Star Wars Ventress Fanfic' is on dedicated Star Wars fanfic platforms. There are some lesser - known platforms too that might have hidden gems. You could also follow Star Wars fanfic writers on social media platforms like Twitter or Tumblr. Sometimes they will post links to their new Ventress - related stories or recommend other good ones.