Most native flowers are over by the end of July unfortunately. Its always a bit difficult to do what you would like and be sure of it as so much depends on the season for when plants are at their best. Elecampane, ox-eye daisy, cornflower (centaurea cyannus) would probably be in flower but I don't think they would give you the effect you are after. I think it might be safer to ask your florist for suggestions as this is a question that does crop up from time to time. You could mix in with some ferns and grasses to give it a wild feel.