Go to your local quarry and ask there.
and find a construction company - something along the lines of lindumgroup or hanson, ask if they have any construction projects on clay land, sometimes they retain spoil for lanscaping, other times they just send it to landfil to layer the tips.
I reckon you'll need between 200-300tonnes, we have about that for sale at a friend of mines farm, for sale at £6 per tonne but delivery will cripple it down to you (its from a village called clayworth
guess why its called that) i excavated a farm track for someone an this is the spoil.
You determine soil type with this triangle
http://en.wikipedia.org/wiki/File:So...reTriangle.jpg , its a lab test to get the exact type though, so i guess you could get an identical copy of a US track if you could get hold of a sample, Harper Adams can do the test for you.
Hmm hope this helps ill see what else i can think of..