I purchased my grids from Wickes in store, I cannot see them listed on the website:
Wickes - Home improvement products for the trade and DIY They came in a pack that makes 4 'cubes'. I wrote down the code and details from the sticker on the shelf, so hopefully if your local store don't have them in they can order them for you.
Details as follows:
Storage Cubes PK4 XT
165125
£24.99
Here's a photo of my cage, so you can see what the grids look like:
http://www.guineapigcages.com/photos...php?photo=9644