It's always a bit weird to me when a writer wants an artist to do unpaid work and refuses to give the artist any reason to believe the story will be any good. I mean, "magical girl" and "space western" are just about the vaguest you can get... So this supposed artist needs to decide he/she wants to work with you for no reward on a manga that may or may not be completely subpar?
I mean, dont you feel like you should be providing people with an incentive to work hard? Manga doesn't draw itself, yknow... You dont just have drawing skills and then make pages appear out of nowhere. It's hours of work every page. So if you're looking for an artist that would work with you, and you don't really have anything to offer professionally speaking, you might as well try to make them passionate about your stories, which right now seem to be vague ideas that you admit to not really having fleshed out. Imagine an artist expecting a professional fully made story from you to adapt into a manga, without showing you his art or at all letting you know what level his work is, and not offerin you anything in return. Then you are expected to actually write out this story, spending hours of your time on something you have no reason to believe will succeed.
So in short... i get protecting your ideas and not having money and every other reason you'd have to expect this from artists, but... Everyone has ideas, almost anyone who's ever seen an anime episode. No one has any reason to believe yours are worth working for. You need to give them that reason, and if that means telling the world a bit of your idea and giving them a sample, then so be it, bedause it's simply rude not to.
It came out a bit more negative than I wanted, cause I'm feeling pretty bad today. I didn't mean to suggest you are purposely disrespecting the artists, just that if you're looking for a serious artist, you should change your approach and try to look at it from the artist's point of view.