You can get the infant size in a jumbo box, but you have to hunt for them online. We have a local store who does delivery and they sell them, but I'm in Brissy so that doesn't really help. Just want you to know they do exsist, but not in the supermarket.
Another option is, if you can hold out changing sizes till she is 6kg, or just buy the smaller bags for a few weeks, crawlers start at 6kg.