I don't know if you can find one anymore, but I got a couple of cast steel Russian anvils from Harbor Freight several years ago for around a 100$ each. They needed several grinding and flap wheels to clean up, but were a huge step up from a cast iron ASO(anvil shaped object). One I used a porta band on and cut the horn off to make a square bladesmith anvil, the other for general work. The biggest thing is to have a good, solid base. You can also make a light anvil act like a heavier one with a heavy base.
That said, after failing to find a good used one, I finally broke down and bought a new anvil, a Refflinghause 330 pound German pattern. Man, what a difference, and what a joy to forge on. That said, I use a post anvil I made from a piece of 4"x4"x12" 4140 for detail work like forging the bevels in.
I've seen video's of bladesmiths working on buried sledge hammer heads, modified rail tracks, stake anvils, ect. It doesn't take much anvil to forge a knife, just means you have to work harder. Of course there's a difference between taking a 1/4" bar stock and forging a blade and taking a 3" billet and forging a knife. That's where a large anvil will come in handy. Also a larger anvil means less fatigue even on smaller stock. Of course the power hammer and press have taken over a lot of the sledge hammer swinging on large stock, but still wouldn't trade my 330 pounder for anything.
