I need to see if there is enough of a market in the U.S. to support this kind of venture, and I'm looking for an efficient way of finding the numbers. Should I just use the U.S. Census? If so, how? I'm looking for info. on how many hjgh school History teachers are out there, how many English teachers and how many government/economics teachers.

easiest way to find out number of teachers would be to contact the Teacher's Union. Try the AFT.

Then - there are generally state wide and national groups which have teacher members - for example - CATE (California Association of Teachers of English).

However - this is not a new idea...there are many sites that offer this. Furthermore, teachers are not rich and there are PLENTY of sites that offer great lesson plans for free. Most teachers care about good TEACHING and are willing to share their brilliant ideas with others. Occasionally it's nice to get paid for the work - but for the most part, we look to help one another.

Great idea but there are a lot of sites available and the lesson plans are FREE...
For example:
http://www.sitesforteachers.com/
www.lessonplanspage.com/
http://www.lessonplanz.com/

So thank you for trying to make a buck off us.

There are already a few websites that do this. They've beaten you to the punch.

US Census Bureau should be able to help. Here is a link directly to what you need.

Hope this helps