This is a little different, but not much, from how to write letter to Senator. Again, this info is from wikihow. There is overlap with the Senator letter writing concept.
I don't think many Representatives (or Senators) are publishing email addresses these days; they tend to offer contact forms on their websites instead. It's understandable as a way to control spam. Just be prepared to compose your letter in plain text, without bold, italics, etc.
I didn't look for others, but my senators and representative all popped up with first try at web search. Maybe they are unique.