Not Official, but RAW. In the description of Eldritch Sorcerer (p. 85), it says:
You can use an arcane focus as a spellcasting focus for your eldritch sorcerer spells.
On page 87, at 6th level, you get a prayer book that acts as both an arcane and divine spellcasting focus, which is when you get your first cleric spells.
So based on the above, you can use an arcane spellcasting focus with eldritch spells and a divine spellcasting focus with cleric spells.
BTW, you don’t NEED a spellcasting focus to cast the spells. A spellcasting focus may be used in place of the material components needed to cast a spell.
