It's a really good idea to look at rare breeds of sheep before choosing the breed you wish to raise, and then join that breed's association. This page provides links for you to do so.
The ALBC is a nonprofit membership organization working to protect nearly 100 breeds of cattle, goats, horses, asses, sheep, swine and poultry from extinction.
RBC is an organization that protects endangered livestock and poultry through preservation and promotion.
RBST, located in the United Kingdom, is a specialist organization dedicated to the support of rare and minority breeds of livestock.