Plum Analytics is a for-profit company which makes a basic level of altmetric information available to users without a fee. Plum Analytics has an agreement with the database Scopus to provide altmetric information within the Scopus database entries, so you can look up either article or author information within Scopus and get access to Plum Analytics metrics, also known as PlumX. The basic metric information is available at this link.
Within the Scopus database, access the record for the publication of your choice. PlumX works best with more recent articles, approximately 2010 and more recent, though there may be some information on earlier articles.