There are lots of futanari out there but I'll just give my recommended ones, here goes -
Tokubetsu Byoutou - only title i know that concentrates on futa
Inda no Hime Kishi Janne ep 2
Discode 1-3
Daiakuji ep 3 and 4 or was it 4 and 5 ?
Ikusa Otome Valkyrie ep 2
Ma ga Ochiru Yoru ep 2 - i suggest you wait for the uncen version
Viper GTS ep 2-3