There are numerous sites and phone apps to lookup chords, but there’s still a point to knowing the theory behind how chords are constructed. And it’s really not that hard. Pick a key and start with its major scale, and then just follow the templates to which notes of the scale are used in the different kinds of chords.
Example:
Major chord: note 1, 3 and 5 from the scale.
Minor chord: 1, lowered 3 and 5.
Then just keep adding on templates to get dim, sus, aug, inversions and any other kind of chord you want. Ultimate Guitar has some really good lessons on this subject:
Lesson 1: major, minor, aug, dim
Lesson 2: 7 chords
Lesson 3: 9 and 11 chords
Lesson 4: sus chords and inversions
