Textile Express is another good one - as well as discounts, they have an own brand range that can have some great things in it. I got some amazing cabbage rose fabric from them for £8/metre that looks like it cost £80.
For big windows, another way to think about is to have fixed panels of a fancy fabric at the sides (these can be just one width each), maybe with a matching pelmet, then have curtains that actually pull in a cheap plain or have blinds that go with the side panels. Roman blinds are also much less fabric hungry than curtains.