Some will do that if it's an idea that's almost guaranteed success, or they're passionate about.
I did it once - and the deal was "30% equity, or £3000 and returned equity if profit of less than £10000 after 2 years". Therefore, he got a programmer for free for the time being (cost 30% equity), and I had a fallback that if his idea wasn't successful, I'd be paid £3000 after 2 years - which was less than the market rate for the time I'd put in. Risk for both parties, but benefits for both too.
If you're unwilling to do a deal such as the above, then what you're saying is "I have this idea that I don't really think is going to work, and I want somebody to code it for free.". No decent programmer will give your time for free - and hire a cheap/shoddy one, and nobody is going to buy your poorly built app.
Also - don't limit your audience. Mac only applications are going out of fashion like there's no tomorrow. Cloud is in. Everyone wants web apps, possibly with desktop versions. I don't just want to be able to access an app on my Mac. I want to be able to do it from all of my macs, my mobile, and from any computer I want.
Also, I'm the type of person that believes not all programmers are great at UX or UI work. That's the work I would be investing my own time into.
Ever heard the term "jack of all trades". If you were refitting your house, you could get a plumber, an electrician, a plasterer, a painter & decorator, and a roofer. Or you could hire a handyman who could probably do them all, but at what quality?
Personally - I'm a programmer. My roots are at developing, UI design is a hobby. UX design, however, I believes comes naturally to both sides. The best way I've found is to have a programmer dealing with the functional programming, a designer deal with the UI, and have the two collaborate on the UX.