have a look on parkersteel.co.uk
Their techincal catalogue is online and will tell you what the stock sizes and dimensions are. That way if you go a stockholder, you can ask for a standard size. You can buy direct from parkers too, but again, it will be 6m lengths plus a delivery charge (from canterbury).
In terms of places to get it from, I would suggest getting friendly with a local fabrication shop, and seeing if they will let you raid thier scrap bin, or buy cut lengths from stock.
hth
Mark