Where can I find a credible comparison between the average amout of
water some major home applicances use (like washer, showerheads,
toilet) and their water-conservative counterparts? (Like a low
pressure showerhead, a toilet that uses less flushes per gallon.)
Any kind of comparison or chart or breakdown would be great. And it
should be from a reliable source, like government statistics.

If you look here:

http://www.epa.gov/watrhome/you/intro.html

and click on Chapter 3 you should find some statistics that might be of use.

Have a good day.