EBay can be pricey, depends where you are, but look in FW or Farmers Guardian for farm sales, also try Farm Trader (think that's the name, auto trader for farmers).
Also ask about amongst local farming community, if you only want to borrow them for a small amount of field: we lend ours to the people over the road or if we have them on the back of the tractor we do theirs at the same time as ours