I do not know of any specific Du'as for repentance. But I also do not see any need for such. Sincere repentance from one's heart and a genuine promise to not repeat such sins and make restitution for any damages caused by our sins Just may be the best Du'as.
Sometimes our own sincere words and genuine asking for forgiveness is the best way.
We can not earn, buy steal or even deserve Allaah(swt)'s mercy, but We can have faith in Allaah(swt)'s pure justice, His willingness to forgive and his infinite mercy.