There is a catch-22 in there. To get more articles you have to get some good ranking. To get some good ranking, you have to have some good articles.
You need to bootstrap the process then and that means creating new articles for your directory. Do not use - even with permission - articles from elsewhere or your site will not get much ranking and therefore wont attract many articles.
These are just guesses, I have never done this, but you could try:
- Hiring some writers from a freelance site. Given them topics and an idea of the size of articles you want.
- Contact people with creative content that are not articles. Maybe someone has a book would like to write an article on the same topic. Or authors of video tutorials, etc.
What I would also suggest is try to get a theme of your directory. This means that the links and contents you get will be more focused and you will have a better chance of ranking for anything.