Celia,
Ideas for writing are something you have to get on your own. I get mine from reading the newspapers and asking, "What if?" I get mine by reading other novelists, even ones I idolize, and think, "How could I have told this story better or differently?" I get mine by going out and living every day and paying attention to what happens to me and those around me.
Judging an idea as "dumb" before it's been put down in novel form is a big mistake. If you condense the ideas for most novels into one sentence or idea germ, they'd probably seem dumb, too. But in the writing, they're expanded and enriched as you add more ideas to them, and suddenly you've got a not-so-dumb novel where a dumb idea used to be.
Just write. Don't judge your ideas before the full story is down on paper, because in the end, the idea doesn't count nearly so much as the story.
If you want published mentors, you can turn to three books that help me sometimes when I'm out of ideas:
Outwitting Writer's Block, by Jenna Glatzer
The Writer's Idea Book, by Jack Heffron
The Writer's Idea Workshop, by Jack Heffron
They'll help you get started, but the most important thing you need is something you can do for yourself: STOP JUDGING THE IDEAS YOU DO HAVE BEFORE YOU'RE EVEN OUT OF THE STARTING BLOCK.
Idea generation is just like every other skill: The more you do it, the better you get. Yes, your first few ideas may seem dumb and trite, but so what? You write those stories, and you'll find out why they're dumb and trite, and you'll improve, not only in your writing, but in your idea generation.
Good luck!
Now get to work...