Is there a good website for cheap fares ex Germany to US?

I am trying to fly Den in late Sept. And I would like to fly LH or UA. Fares for flights from the UK on both these carriers are not good.

Have had a look a Galileo and the availablity on the UA (codeshare) seats on the FRA-DEN look much better than LH flight.

1 FRA DEN 1320 1545 LH 446 F4 A4 C6 D4 Z2 Y9 B9 M9 H0 Q0#340*C 2 FRA DEN 1320 1545 @UA8878 F9 C9 D9 Y9 B9 M9 H9 Q9 V0 W0#340*C

Any ideas if I could fly ex Germany ?