Where are you from that you can not find any good materials?
Oak is the best (or is that the most used) material for ribs. Other than that, its all up to you what you do. If I were to thin anything, I would route the inside edge of the top, above the mortises. But then again, for the amount of work and weight savings, I wouldn't thin it. Unless you are perfect at making mortises, you will greatly increase your chances of breaking one by thinning the wood. Not worth it.
Hey Will. I have a Yost with nice brown cedar that shows through the skin. Its quite ugly to be honest. Unless you use clear skin, its not worth trying to show off your frame through the skin. The unfortunate thing about making SOF's is that you make something incredibly pretty, then cover it, but get something even more pretty when you are done.
Scott