Free: Contests & Raffles.
Rio BlueSteel in #2. Cheap, deadly.
I shot steel one time and switched to premium non tox after watching steel literally bounce off of the bird and watching it fly away unaffected.With the cost of everything else to get there and back, plus generally low bird limits, shotshell cost is a very small part of the equation. Particularly when there are times you may only get a shot or two. My faves are HeviShot, then Tungsten Matrix, then Bismuth, depending on the shotgun and type of bird.
I just bought a 28 gauge and cant wait to start hand loading TSS duplexed #4 steel / #9 TSS. I've shot a similar load out of a 20 gauge and was shocked at how well it performed. Bismuth is great but you have to shoot a real tight choke, like extra full to maximize its potential. And if you shoot a benelli the BOSS will require a new hammer spring.