Violet is a dominant trait, so all budgies that have a violet gene should show a little violet. However it can be hard to see because it looks a lot like a dark factor unless it's a 'visual violet'. If you see any violet feathers on a blue or green bird, the bird is probably violet. Usually you see a little violet on the side of the neck, at least on blue birds. Visual violets are sky blues with double violet factor, or cobalts with single or double violet factor. I've seen sky and green violets at Petco, so they're not that unusual. Getting a really nice violet can be a little hard though and for that I'd seek out a breeder.