I think you are referring to Datura, (Angel's Trumpet) which is commonly available by seed and also small plants. Just search the web for Datura and you should come up with a ton of sources. There are also what used to be called "Tree Daturas" which were separated into their own genus, Brugmansia, but these have a shrub or tree form and don't come in purple, while Datura does come in purple and is a much smaller plant, an annual i think. However if you are referring to moonflower that is different, there are a ton of threads about moonflowers on the front page of this forum that list sources for that.