fabric 101 and wikipedia is also good. Most basic sewing books from the library will talk a bit about fabric.
Lots of ebay shops sell affordable fabric, but the thing to remember is that fabric is not cheap, I would be surprised to pay less than £5 per metre, and average affordable (i.e. not expensive) fabric ranges from £7 to about £16 per metre. Apholstry fabric is more expensive, as are wool, designer, and other specialist fabrics.
Ikea is brilliant for very cheap but good fabric, with prices starting at about £1.90 per metre, so if you can get to an Ikea, stock up!
Remember also in most fabric shops you don't have to buy a whole metre.
Also, keep your eye out for other sources of fabric: old clothes, curtains, bedlinen (cheap Ikea duvet covers give you a lot of fabric for your money); charity shops (old clothes, men's shirts, curtains, table cloths etc. etc can all become fabric supplies).