There are countless opportunities for sourcing grants and a simple web-search will reveal many potential funding opportunities.
However, in the field of education and research there are several institutions and awarding bodies that are excellent places for any applicant institution to begin their search for grant funding.
Foundation Center
Knowledge to build on (training)
The Foundation Center is a clearing house of grant making bodies, grant writing knowledge, and general grant funding information and should be a first stop for anyone beginning to source a grant or write a proposal in education and research:
http://foundationcenter.org/getstarted/
Grants: Beyond the Money by Alan November
Solid background information regarding grant writing and the grant award process is important. You should read as much about the general process as you can. This article by Alan November is a good introductory article to the grant proposal industry:
http://www.anovember.com/default.aspx?tabid=159&type=art&site=182&parentid=26

California Department of Education
Finance & Grants Division
Every state funds and seeks sources of grants. However, some states have collected better resources and information than others. California’s Department of Education, Finance & Grants Division, is an excellent source of grant making and grant writing information:
http://www.cde.ca.gov/fg/

Bureau of Educational Research
College of Education at the University of Illinois at Urbana-Champaign
The College of Education at the University of Illinois at Urbana-Champaign maintains a Bureau of Educational Research which offers grant funding opportunities as well as grant writing information and resources. This site is an excellent resource for the would-be grant writer:
http://www.ed.uiuc.edu/ber/webpages/Grant_Writing_Guides.html

National Science Foundation
Research and Education Grants
The National Science Foundation is a good source of funding opportunities and should be explored:
http:www.nsf.gov

National Science Teachers Association
Research and Education Grants
The National Science Teachers Association offers much information about grant writing as well as functions as a clearing house for science and education related grants such as the Toyota TAPESTRY grant program:
http://www.nsta.org/

SRA International
A Professional Society, Educating and Supporting Research Administration Around the World
SRA International is an excellent source of international grant opportunities in education and research. It offers not only grant funding opportunities as well as grant writing guidance but also is a source of grant writing career related information:
http://www.srainternational.org/sra03/index.cfm

U.S. Department of Energy
Office of Science Grants
The U.S. Department of Energy makes grant awards in science and education. It offers a wealth of grant funding opportunities: 
http://www.sc.doe.gov/

U.S. Department of Education
Office of the Chief Financial Officer
Grants and Research Information
The U.S. Department of Education is another valuable resource in grant funding opportunities. It funds many different types of programs and projects in education and research as well as offers instruction in completing grant proposals:
http://www.ed.gov/about/offices/list/ocfo/grants/grants.html

Carnegie Corporation of New York : Research and Education Grants
The Carnegie Corporation is a private non-profit institution which awards many grants throughout the year in a variety of fields and disciplines including education and research.
http://www.carnegie.org/

Rockefeller Brothers Fund
Research and Education Grants
The Rockefeller Brothers Fund is another private foundation which awards a variety of grants including education and research related projects throughout the year: http://www.rbf.org/

These and many other sources of information are available in the web. Again, as I mentioned in the misconceptions about grant writing processes, the research and planning phase is probably your most important activity related to grant writing.

After all, if you have done your homework then the funding agency you petition with your grant proposal is likely to be extremely interested in your project or program because, ideally, it is perfectly aligned with its own mission and objectives.