I found out about this via the ALA Federal Library and Armed Forces Library Roundtable ListServ. Science.gov is a portal to various governmental databases and technical information sites. It might make it a little easier to locate governmental authoritative sources.
I looked under the category of Science Education. There are links to sources from the Library of Congress, National Science Foundations, NASA and so many more agencies that have science data.
You can enter a search term or you can use the advanced search to find by keyword, title or to select just the area you want to discover.
Science.gov is a great help in finding data but I do want to mention a concern. I followed a group of links about chemistry clip art from the Food and Drug Administration. Many of the links were dead or long gone.
Not blaming the FDA for this, external websites change all the time. Part of the responsibility is trying to maintain quality links so you do have to go back and see if a source listed in 1999 is still viable in 2009.
Science.gov is a cool resource that can help you locate governmental science related materials. I'll spend a little more time checking it out.