Looking for a good source of straw

I am looking at no-dig potatoes next spring and i plan to use a mixture of well rotted manure, my own compost and bale straw to make up a bulk of 6 inches to cover my seed potatoes with. then as the sprouts start coming through I will shut out the light with lawn mowings and path fabric or cardboard if I need to.

What I am short of is a cheap source of bale straw and what straw it should be - wheat, barley, lucerne?

Has anyone else tried this and what did you use?  What prices am I looking at for straw bales?

£2.50 in rural Hampshire so it's probably the same in Cornwall and your best bet is a friendly farmer watch out for straw harvest when they're loading the carts and go and have a chat, you never know.