The best place you can go to is your teacher. They should have a large collection of monologues to go through. After that, I suggest:
1. Local Library
2. State Library
3. Write your own, inspired by a book, movie etc.
Here's a recent one I've just found.
Script Archive: Monologues
Basically, my advice to you is to just keep reading as many as possible. If you find something that could work, you can always edit it to suit your skills.