I get mine from either Phoenix Steel in Sheffield or from Cromwells (branches all over the place)
Phoenix Steel..
http://www.toolsteels.co.uk/pricelists.htm#
Cromwells..
http://www.cromwell.co.uk/index
In both cases, what you are looking for is Ground Flat Stock.
Good luck with the blade making. I'm just getting into myself and whilst it's great fun it can prove to be pretty annoying when you think "I'll just grind a little more away just there...." and you catch it wrong and stuff the job right up! Aaaargh!