FXCM and a few other similar brokers have the 3 main grains, one of them (Oanda?) has Bean Oil or maybe Meal but that's ablout it. If you're in the UK, IG Index has all 5 of them via spread bets but the spread is wide so only good for longer term trades. IG might offer them on CFDs, check with them.
As jbt says, there's not much retail interest in the grain complex.
Oand
Oanda has soybean and wheat of the grains
But if I am not completely off, there are restrictions for US citizens that they cannot trade those kind of instruments?