Pre WWII, almost every farmer had an anvil. A lot of anvils were sent for scrap iron to help the war effort. Post war, as farmers quit farming, their anvils were sold off. That supply dried up over the years and it is difficult to find one in a farm auction nowadays. My ex was a farrier and bought old anvils whenever she saw one. She would restore them and sell them. The anvils that I have, with the exception of the two mentioned above, were remnants of her collection.
When I last checked, used anvils were selling for around $4/lb. And that was regardless of condition. You can buy new anvils from Centaur Forge in Burlington, WI, They have a fairly good selection in both size and style.