Although not a reliable and continuous source, I've had marvellous luck with Op-Shops! Some friends once found some old, deep red velvet curtains in one and I'm forever coming across "suitable" flannels and cottons.
You can also pick up clothes up to very large sizes - I tend to the larger sizes in stores like Rockmans and Suzanne Grey, but have found pieces much larger than I'd need, that can be unpicked and restyled.


Also, try an Ebay search for fabric?