There are so many different ones to choose from.

The first one i got was a cheapie from BigW that lasted about 6 months before all the internal lining ripped out. The next one i bought was a dearer one at $80, Baby Taft brand and I still have it. It is faded, stained and starting to wear out, but it was the best nappy bag I've ever had.

You can get back pack ones, ones that fold out, ones that come with the pram. Just have a look at them and see what you think will suit your needs better, like if you go out, are you out for the day and need lots of room for nappies, spare clothes etc. Also some don't come with very good bottle holders.