I just got a purple one from argos. They seem to be the cheapest place. A bright pink one was brought down first, but you can ask to see them on approval first and I swapped it for the purple one. (Didn't ask whether there was a white one).
They aren't all white. In the review on argos it says that they had to have their shocking pink one swapped for a white one. There are a few different varieties.
I know it would make life more difficult for the stores, but I wish that each variety would have its own code so you could get the one your kids wanted and know whether the one you wanted was in stock or not.
They are all white. The ones you see in adverts that are coloured are just examples of what they might look like when the child has decorated them. If you look again, with that in mind...you will find some.