Where can I find Dominant seventh chord (e.g. C7)?

The quickest way to find dominant 7th (e.g. C7) chord is by using Chord Dictionary screen and taping on quick filter “7” as shown over here:

You can use the same filter to find major, minor, diminished, augmented and 7th, 9th, 11th, 13th chords. If you didn’t find a chord you can use “New Chord” option to create a new one.