I've written a few scripts for film, but think I need to collaborate with someone. I always run out of ideas. Where would I find a co-writer?
A snappy sarcastic answer is tempting... Something like, "How about Craigslist?"
But on second thought...
What you're looking for is for someone to solve your problems with writers block for you, rather than solving them yourself. Writers block is something that all writers have from time to time. There are a number of questions here that address it that give ideas on how to solve it. Try searching for questions with the tag, "Writer's Block."
But if you're really stuck and really want someone to help you, start with the people you know best. I always talk things through with my wife... She's great at coming up with ideas that help me through. Also try taking a break, exercising, watching a movie, reading a book, or whatever gets your creative juices flowing again.
Writing is sometimes easy, and it's that easiness that makes the times that aren't easy more frustrating and difficult, but it's also those times when you keep on writing even when you think you can't that you'll grow and become a better writer.
There is a website specifically for finding co-writers, http://co-writers.com . It's not hugely active, but it is free, so it doesn't do any harm to try it out.