What is the difference between these two plants? I have seen white, but I know there is also a purple. Where does one buy them?
Solanum laxum or solanum jasminoides - potato vine
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/
I have read that solanum jasminoides is purple, solanum jasminoides album is the white form. I don't know what the difference is between solanum laxum and solanum jasminoides album.