The Credencial del Peregrino is probably the most important piece of equipment for your pilgrimage. It is your ticket to most hostels on the way as well as the proof of your pilgrimage on the Camino de Santiago.
On your way you will collect sellos (stamps), which which will entitle you to stay in an Albergue (the pilgrims only hostels), if there is space that is.
You will also need this passport to prove that you have walked the last 100km (or 200km by bike or horse) to receive La Compostela, the certificate of accomplishment that you can get in Santiago de Compostela upon completion of your pilgrimage.
Below is a list of some, not all, places where you can get your pilgrims passport.
American Pilgrims on the Camino
Irish Society of St James
Canadian Company of Pilgrims or Du Quebec a Compostelle
Confraternity of St James
Confraternity of St James in South Africa
Australian Friends of the Camino
La Société Française des Amis de Saint Jacques de Compostelle
Deutsche St. Jakobus-Gesellschaft e.V or Region Norddeutschland in der Deutschen St. Jakobus-Gesellschaft e.V or Fränkische St. Jakobus-Gesellschaft Würzburg e.V.
Confraternity of Saint James, Norway
Nederlands Genootschap van Sint Jacob
Sankt Jakobs Bruderschaft Ă–sterreich
Górnośląski Klub Przyjaciół Camino
Vlaams Genootschap van Santiago de Compostela or Association Belge des Amis de Saint Jacques de Compostelle
Les Amis du chemin de Saint–Jacques de Compostelle Suisse
Confraternita di San Jacopo di Compostella
Associação de Confrades e Amigos do Caminho de Santiago de Compostela–São Paulo–Brasil or Associação Brasileira dos Amigos do Caminho de Santiago
most of the Albergues and Pilgrim offices issue the passport