The ANSI Webstore will let you search for standards from many standards publishers (including but not limited to those linked above) by keyword or document number. Keyword searches may give results from multiple sources including standards that are freely available on the Web. The left side of the page provides the ability to browse by publisher or view all standards developing organizations in the database.
IHS is one of several companies that sells copies of industry standards. The site provides an excellent search engine to standards of all kinds and will often mention whether a particular standard has been superseded or revised more recently. Searching by title keywords will help locate information on whether a standard has also been issued by another standards agency.
Primarily an index of journal articles, conference papers, and technical reports across all engineering disciplines, and available to ISU affiliates and on-campus users, Compendex also includes records for some standards. The current list is AIAA, ASCE, ASTM, IEEE, SAE, and SMPTE, plus a few scattered standards from other standard developing organizations. Note that this does not mean the full text of the standard is included or available, but this can help with locating standards of interest. Choose "Standard" as the Document Type when performing a search to find standards by a particular organization or on a given topic.