Sometimes, in my area, we can get flat bed semi loads of old railroad ties. Some will have some damage to one of the ends, but most are great for posts. Heavy! And tall, even the 10 foot ones are plenty tall.
Besides using them as posts, I couple places I've rode on, made large round corrals completely out of ties. They had really good supplies for them.
Others use ties as posts and cable between the post corners. You can buy cable used, too. Some people drill through the ties and run the cable. I prefer to buy keepers to run the cable through. Drilling through railroad ties can be tough.
Even though the ties are heavy, I'm 5'4" and not stocky and I can work a tie into the ground and set it.
Oh, and if you are near any oilfield work (PA, OH, TX, CO, WY, MT, ND) you can usually find used oilfield pipe for sale. Again, used cable works well with that also.
Auctions are sometimes good ways to get building materials, but I don't have the patience for auctions.