find a bank that sells ace seeds' oldtimer's haze. it's a true purple haze and even has a really cool sounding blackberry liquor flavor. i want to cross that with DJ shorts' grape krush myself. some of the best sources for pure sativas are ace, gage green, cannabiogen, world of seeds and holy smokes, but purple sativas aren't too common.
killing fields and jack f7 are sativa dominant hybrids. i just got my pack of jack f7 the other day actually and can't wait to test it. if you want the trippiest and most old school buzz, you should look into 14 week plus strains, but there's a lot of great hybrids that are more practical in the 9-12 week range.
hempdepot, as mentioned is a good canadian bank i used to buy joey weed seeds from all the time. they have a great selection, but seem to specialize in more expensive breeders.