Flax is another name for linseed, you can get it from most feed merchants. Some info HERE
I buy it ready cooked as 'biscuits' about 2cm square and add it to my mares feed. It is high oil so provides energy and is good for the coat. You can also buy linseed oil and add that to feed. I think it is supposed to help with other things, but can't remember off the top of my head...