Do you have any suburbs near you with a high Indian or Pakistani population? If so you could probably find some there or at least get the lowdown on where to get them from.
If you want to buy online try the modest clothing directory Modest Clothing Directory (formerly Dara Marie Modest Clothing)
I've never had a drama with any company on their directory.

I loved the maternity clothing I bought from egg. They're a NZ compmany but you can get their stuff in Australia and online. It was really comfy and well cut. It was expensive but it wore really well.