The credencials are not particular to individual routes. You can use one from your national pilgrim association, a Spanish one (available from Ivar's online shop), a French one (email@example.com), or from anywhere, really.
You can get it from the Presbytere - 12 Rue du Cloître, 13200 Arles, France. You'll get the stamp for the Diocese of Arles and Aix. The building doesn't look like an office - but if you ring the bell someone will come and let you in