look up steel stockholder in the yellow pages - most of them are used to people going along and buying odds and ends, although how you get it home on the bus...
curtain poles look clumsy over 30mm in diameter - and, as you say, timber poles sag - and, unless you use unsustainable hardwoods you have to wax them to keep the rings sliding.
If the window is eight foot across, why not have an elegant bracket to hold up the pole in the middle? - John Lewis do them. They also sell brackets and rings that allow the rings to pass over the bracket
I'm interested by Mr. Pig's post. 'Flat' arches should have a slight rise in them to counter the impression that they are sagging. Bricklayers used to know that. Very few architects do.