I have used a plain old fluffies from the salvos for 50c and it worked beautifully. You can get them from K Mart I think, and they are about $12. They are just a little lined pair of speedos. Keeps the poo in, and thats all they do.
I also have a pair of funky bumz from NZ, and these are really great. They are lycra on the outside, and microfleece on the inside, and have double gussets. They were about $18 to buy I think? really comfy for jenna, and they looked very cute.
I have just checked the site and the current size we have is 9-18m. Jenna is about 11.5kg, so I will try it on her tonight and let you know how the sizing still is for her.