BCSE has one that works for these versions : X-cart Versions 3.5.x, 4.0.x, 4.1.x, 4.2.x, 4.3.x, 4.4.x, 4.5.x
http://www.bcsengineering.com/store/mobile-mod-for-x-cart.html?&partner=marketplace
I'm planning an article on this very subject soon. There are a lot of mobile options under development as we speak, so you may want to wait just a little bit before you make your decision.