I don't think there's a legitimate way to obtain them, other than either buying the module itself (I don't know how much it goes for on the secondary market), or maybe finding someone who's selling just the scenario cards (and even this is a little bit fishy; who's going to sell just the cards and not keep copies?).
It might make sense in these days of everybody going digital, to sell scenarios digitally, but somehow it would mean a pretty big change in MMP's (or any TPP) business model. I mean, currently I still have big binders full of scenario cards (and copies of scenarios from magazines), but the reasonable way to go for most players is to scan everything and have their whole scenario library on a memory stick or whatever. The problem, I guess, from publishers' point of view, is that they're afraid that players will stop buying their products and just share the files.