I've never managed to get more than a mile from a metal shop. Have you tried looking for steel stockholders in the yellow pages. Otherwise try steel fabricators if you don't need the full 2m * 1m - just raid their skip and give them a fiver.
On-line shops tend to be expensive as they have to add postage costs, and metal isn't the lightest of materials. I've never ordered online so can't actually answer the original question.
Now would be a great time to find a local fab shop, (better than a dedicated stock holder imo as they will cut to size.) Pre xmas clearout, everything will be chucked in the scrap bin! 8)