Supreme Court opinions can be found in many places including supremecourt.gov
Find federal cases brought before a District Court or Circuit Court of Appeal. Generally, circuit court websites follow the form of www.ca<circuit number>.uscourts.gov, as in www.ca9.uscourts.gov for the Ninth Circuit. District court URLs are more variable, see the U.S. Courts link below.
NOTE: State trial court decisions are not published. Access to state court documents varies widely between states. Checking the state's judiciary website is a good first step (search for county + court record search). In cases where records aren't digitized, call the county records office to inquire about mailing or faxing. Some courts may charge a document fee.