An example of a method which gives the party access to free resurrection magic is casting True Polymorph to become a Planetar. Planetars can cast Raise Dead 3/day without paying the material cost.
This is not a duplicate of Are there any spell casters that can cast life giving spells without 'expensive' components? because I believe there are more methods of getting access to free resurrection than simply being these spellcasters. Also, the methods by which players gain access to those spellcasters may not exist within the system and may need the story to provide them.
What ways exist for a party of adventurers to get access to free resurrection?