Hi Kylie,
I used a Salomon XA20 with a 2litre bladder. It was ideal for me; plenty of storage for the mandatory gear, side pockets for gels, trail mix and vegemite sandwiches. Also didn't need to carry more than 2 litres as the checkpoints are just close enough together to fill up as you go. Best source, as Regan suggests is
www.wiggle.co.uk. My Partner, Jaci just got the Ladies version of the XA20, which she ordered prior to TNF but they ran out of stock. She ran with a Camelbak Mule, and she wasn't really happy with it. Hope that helps too.
Dean