You could get away with a lot of the non-maternity stuff in the likes of Fat Face and White Stuff, I would think - and a lot of the stuff there is pure cotton. Obviously it depends on your shape and size, and on the cut of the clothes. But I was a size 10 pre-pregnancy and despite putting on a terrifying 4 stone (!!) I'm still wearing some non-maternity Fat Face tops, in a 12, at 40 weeks.
It's annoying isn't it? You don't want to spend fortunes on clothes you won't wear for very long, but viscose etc are a bit grim. I'm not a lover of clothes normally, and I tend to just wear basics, but I definitely prefer natural fabrics.
I'd like plain maternity clothes in bright colours, made of cotton, that don't cost a bomb. Can't really find them! It doesn't help that I'm a fat bastard, so not a lot of mileage in buying bigger sizes of "normal clothes"!