|USA.gov searches the Internet for '.gov' and '.mil' domains as well as state government web sites. Start with a keyword search and scan the results to see which agencies are concerned with your topic.
The library catalog, Search It, is a very good place to start searching for government documents.
IMPORTANT! Use the Advanced Search to specify the government as the author. Here are some examples:
Note that you can also filter by Government Documents as a Type in the Search It catalog. This will bring back government publications at differing levels of government.