Used copies of Ray Dream (almost always Studio, which is all right, because it's better) are sporadically available on places like Ebay. All in all, it's almost infinitely more rewarding to use as a modeler than Carrara, but isn't always as good a rendering tool (though some effects are done better in original RDS). Personally, I recommend trying to get both and using them as a single program broken into parts, something like a cheap Lightwave. It's a pretty decent pair, and still infinitely cheaper than, say, MAX.