Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
Almost all outdoor shops have them. They are called Ponchos. I bought mine in Open Air for £30 last year. They are very useful but easily tear in high winds. Get some mini caberinas so you can stop it flapping.