I usually Google Safco or Seville grid storage, I've managed to get the Seville grids for £25 before.
I recently bought some 4mm correx from ebay This one is for x 3 correx though, you could always ask if the would sell you one... also I read somewhere else that Wickes sell the same sort of thing, it's only 2mm but it's cheap enough to get 2.
I thought I was going mad, glad you got the gist of that! I can't remember what I paid, but got earlier this year. However, I've just remembered that I thought I'd got a good deal, but hadn't realised you don't get grids for the base. So, yes, the name we must not mention are probably not the cheapest!