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.