There is nothing in particular that you should use.
Consider something in your price range, mostly.
But, you want something that is not too heavy and non porous. Consider a size that will fit well in your viv, maybe a size where you don't have to cut them to fit, or at least, not cut a lot of them..
My fiance works for a company that makes tile and he just brought home some waste/recycle tiles. They were just ceramic.. or porcelain. I can't remember. Check out what's on clearance at your local hardware store
