Yarrawin, I didn't even know you could buy singlets that had a cup size. Can you get them in FF? Where were you looking at them online?
The ones in target and kmart are by cup size but I haven't seen any online. However I've seen posts on other forums that have come up in my googling from size 22 women who are comfy in a size 18 Loveable Yummy Mummy Cami...

Jaki - Thanks for the tip on the ebay wording... I will take another look without typing in the size!