I'm kicking myself now for not checking at Costco before we booked our honeymoon! They don't do just flights, and the destinations are a little limited, especially if you are looking at Asia. But, they have a lot of options in Europe and tropical locations. They sell cruises, tours, hotels, etc. I just booked a trip to London that includes a 5 star hotel, private car transportation from the airport and $400 in tour credit from Costco. When I priced it out separately- just the flight was as much as the entire package through Costco!
This is definitely a good place to at least start your planning.