Quick question -- if I'm a M/15.5 in US, what size YSL shirt do I buy? I'm thinking 39, but for some reason I've got it stuck in my head that I'm a 42 (probably because there's only one left, and it's a 42). HELP
Well, your question doesn't really make a lot of sense, which is why you're getting that sort of reaction.
As somebody else pointed out, almost all jeans are made of cotton. You could buy pants cut like jeans in some other material, but they won't really be "jean" as most people understand the term. And "stacking" is generally a product of the length of your hem. If you don't want stacking, just hem them appropriately. (Lots of people actually like the aesthetics of stacking.) And being skinny has nothing to do with it.