ask the human resources department at your employer. Federal law requires statements to be sent to you.
Your best option is contacting the Human Resources department even if you left your employer a while ago or the company has merged or moved. They will have record of your employment and know which qualified plan you may have been enrolled under. It's not uncommon for large companies to have legacy plans leftover from before a company merger. Sometimes these plans are converted over into the current one. While this may not be the case for your situation, it's something to consider for others who this impacts.