where can I get farm manure from?

I am getting my garden redone - it's extremely heavy clay with no organic matter in the soil at all. I have been told that well rotted farm/horse manure would be good to help break the clay up - does any one know where I could get some? Annie at the Walled Garden nuirsery said it should be at least 6 years old, and a minimun of 3 years old otherwise it will be too concentrated in clumps still. I live in Denholm - near Hawick.

Thanks for suggestions,

 Monica