Kent Tungsten Matrix is suitable for tight fixed chokes, and while it is not cheap, it patterns well and kills ducks and geese like the best lead loads. Expect to pay $3.00 per shell for 2 3/4".
Bismuth is the next best option and is available as loaded shells, and as loose shot for handloading through Precision Reloading. I've shot a lot of it and it responds to chokes just like lead and kills ducks dead in the air.
Niceshot is another highly regarded lead substitute available through RST as loaded ammo (lighter, lower pressure loads) and as loose shot. ITX is Ballistic Products' new offering and their claim is that it is malleable enough for old barrels and is very effective. I have not used either of these two products, yet.
None of this stuff is cheap. Handloaded ITX shot 1.25 oz loads would be the least expensive, about $1.75 each. However, the first time one of those loads kills a "can", greenhead or a fat Canada goose dead in the air at 50 yards, you'll know it's worth it.
