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I think they're synonyms for the same plant (at least that's how they're listed in Plant Files). http://davesgarden.com/guides/pf/go/2078/ The white one is available everywhere--pretty much any nursery you go to, or even places like Home Depot, OSH, etc would typically have it. I don't think S. laxum/jasminoides comes in purple--there are plenty of other Solanum species that are purple though but I don't know which one it is that you've seen. Lycianthes rantonnetii is a fairly common purple one--it's not considered Solanum anymore but looks very similar and I see it a lot in gardens/garden centers so maybe that's what you've seen. http://davesgarden.com/guides/pf/go/31441/