I now use either Small Pet Selects or Mile Four's oyster shells. I haven't had any problems with those brands. My ducks seem to prefer small pets, but when the oyster shell stocks got low they did eat the Mile Four's just fine. I think mile four's is actually an alternative to oyster shell with...
Be careful with those. I found actual shards of steel wool on the one and only bag of Manna Pro's 50lb oyster shells I purchased. I'm not talking just one piece the bag was absolutely littered with pieces of metal. I don't know if it was a bad batch or what, but never again will I allow those...