Official articles of impeachment are voted upon by the U.S. House of Representatives. Look for House Resolutions and House Reports at Congress dot gov.
You can find them in print in large law or government document libraries and usually, though not always easily, online. Some online Congressional research resources are fee-based databases and some are free.
For example, see previous post from September 26, 2019: What Does an Article of Impeachment Look Like? Read Presidents Nixon and Clinton Articles