In no particular order:
http://en.wikipedia.org/wiki/Horse http://www.imh.org http://www.equiworld.net/global/faqs/ http://www.ultimatehorsesite.com http://www.equisearch.com/ ...
Horsemart is great because you can put in a height range, price range and where you would like to travel to, to fetch the horse and the colour you would like. Another good website is adhorse because...
In terms of the most popular breed, the Quarter Horse Association has the highest number of registrations. In terms of most money, top Thoroughbreds routinely go for millions of dollars.