Then I doubt you have looked very hard. However, if all you have done is use search engines rather than starting at a reliable source I can understand a lack of success.

Here is one simple trail:

start at a major spacenews source http://www.msnbc.com/news/spacenews_front.asp

scroll down to the section mentioning asteroids (More of MSNBC’s greatest hits The greatest hits from the Hubble Space Telescope ... what’s up with asteroids ... and other highlights from space.) and click to go to:
http://www.msnbc.com/news/spacelib_front.asp

Scroll down to the VERY FIRST impact risk article mentioned and click to go to
http://www.msnbc.com/news/293872.asp

Using the links there you will have access to many important asteroid sites worth bookmarking. The Ames site (Asteroid and Comet Impact Hazards) also has links to numerous detailed reports on the impact hazard. The link is out of date however. The current link is http://impact.arc.nasa.gov/

key links there include:
http://impact.arc.nasa.gov/related/links.html list of key links
http://neo.jpl.nasa.gov/ JPL's site
http://impact.arc.nasa.gov/congress/index.html Congressional reports
http://impact.arc.nasa.gov/reports/index.html Professional conference reports
http://www.nearearthobjects.co.uk British report (most recent detailed report)

Another link (Asteroid and Comet Impact Hazards) back at the spacelibrary page goes to an article with a broad overview of the impact hazard which has numerous Tunguska links. http://www.msnbc.com/news/373938.asp

Regards,
Mark Eby