top soil

new to all this but we have set aside a small bit of garden to grow some veg. i thought it would be fun for my 3 year old son and a way to get him to eat more veg. busy building a raised bed and need 1 tonne of top soil, how much should this cost and where can i get it (we are in the northeast). the bed is going on top of clay do i need to do any prep to the clay? we also have 4 bantam hens can we muck them out and add strate to the soil?

cheers
dave

i'll have loads more questions mind!