I agree, 5x0 outfits are hard to find.
Target have terry toweling all-in-ones (similiar to Bonds Wondersuits) but yes, finding 5x0 in the colour you want can be tricky. I just keep popping my head in when I am down the shops to see when they get more stock in. I think Kmart also stock 5x0 but like Target, it sells quickly.
I've been told Mothercare have a great range of premmie baby clothes, but haven't been into the shop myself.
Bonds is a smaller make, so I have a few 4x0 Wondersuits which I think look around the same size as my 5x0 outfits.
A friend was telling me that Best & Less have a premmie baby section as well - haven't been in yet myself.

I am expecting fraternal twins, we found out at our 20 week scan that our twins are both girls. So we will be a family of 3 daughters - poor DH, he won't see our bathroom for the next 20 years! I am a surviving fraternal twin, my Nanna had fraternal twins and my Aunty also had twins ... so I guess you could say twins run in our family.