Pefferlawchicken, go to farm auctions (look in the paper or on auction websites for schedules, we're just starting to get into that season). Or you could try the 'farm implements and tools' auction at Vanhaven Arena in Uxbridge next month, although I would warn you you'll probably be bidding against me for wire fencing unless i know who you are in the crowd LOL. Obviously it is hard to be certain what a given auction will have -- although the auction bills and online listings usually mention if they have a lOT of fencing, there are often unmentioned scraps, but sometimes not -- but I have gotten most of my fencing that way for WAAAAY less than retail.
If you pass a farm or junk-filled property having a garage sale they are also worth stopping at.
Also, try kijiji.
Good luck, have fun,
Pat