I agree with everyone above. Wilko, Argos and Ikea are good places to check. However, not all local Wilko's stock fleece (mine doesn't).
Here's a link to some fleece at Argos (£12.99 - buy one get one free):
Buy Satin Edge Fleece Throw - Chocolate. at Argos.co.uk - Your Online Shop for .
However, I just bought a large cream fleece for £3.95 from Poundstretcher. My cage is 2 x 4 with 6 inch correx up the sides, and the fleece goes right up the edges so I can tuck it. Perfect and cheap!
Store locator here:
...instore - SHOP SMART, LIVE WELL.