I've come across a few independent curtain shops who have a 'please help yourself' box outside the shop. So it might be worth checking local curtain shops to see if they can offer you anything (either for free or for a very small payment).
Otherwise, of course, the obvious place to look for cheap fabrics is your local charity shop. Nearly all charity shops regularly send large quantities of unsaleable fabric items to recycling companies. They might only get paid a few pence for each bin liner full of fabrics but that's better than having to pay to have them collected as waste. If you tell the manager the type of stuff you're looking for, he/she will probably be happy to put items to one side in return for a small donation.
Even if the charity shop has already decided that a fabric item is saleable, it's unlikely that you'll be charged very much for it. I recently needed a door curtain, which had to be of a thick material, lined and the right size to match the dimensions of my door. It took me less than 10 minutes to walk to the nearest charity shop and back. I got exactly what I wanted for �3. I also noticed that they'd got loads of fabric samples (suitable for your requirements) between 20p and 50p each.
Chris