Serrano ham is a type of ham done in Spain. Usually you let the leg of the pig dry in salt. When coming from ordinary pigs is just serrano, but the most appreciated is the one coming from black pigs that are fed only with acorns.
If you really want to know more, you can check my blog about Jamon Bellota (Acorn Ham) or my web about Spanish food: http://jamonbellota5j.blogspot.com/, http://www.elcapricho.nl
Regarding where to buy it, if you are from US, a place called La Tienda will ship it to you.
Hope this helps,
Have a nice day!