The one place I found that has the best prices and the functionality you are looking for is “American Forces Travel” which is operated by Priceline. Unfortunately only certain people/groups are eligible to use it (I only bring it up because you can get really good deals if you are eligible to use).
I bought my Delta flight last week for next week and with what was quoted on delta, google flights and other websites, I got my price down by 600 dollars.