In my area of southern Iowa and northern Missouri there are a few fencing companies hired by farmers with drop off places at lumber yards or grain selling and storage facilities.Hundreds of posts can be there.If looked at soon enough the wood wasps won't get in them.
Individual farmers will take out and redo whole fence lines occasionally that are overgrown with hedge along an old fence line and have them for sale on their residence also.Our fence lines here are mostly hedge.
Those you posted are pretty cheap.A straight 12" corner post can go for as much $75.00 here.Still worth the price if making bows is the goal with the use of them.