Here are some helpful links that provide 
good information about the breed

http://www.wcmhr.com/abcsofminis.html
http://www.theminiaturehorse.com/article/conditioning.htm
http://www.blackdraft.com/aboutthebreed.htm
http://www.falabellafmha.com/

The following links will take you to the two main registries for the miniature horse.

http://www.amha.org/
http://www.shetlandminiature.com/