They are quite hard to find in the small sizes, I have been looking all weekend for when we go on holiday. Mothercare have 2 in this size, I had to order online though as they had none in the store. And I also got 2 from BHS in the sale, and one from the Disney store.
Be careful with some of the sizes though, my LO is 15 weeks and only weighs 12lbs but 3-6 in most shops was too small, had to get 6-9! xx