Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: Fire resistance makes the player immune to fire and lava, and as such makes the player immune to blaze's ranged attacks, (but not their melee attacks.) The only source of fire resistance potions before entering the nether is witches: if they are set on fire they will begin drinking a fire resistance potion, and if they are killed before they finish there is a small chance they will drop it, making them farmable. A bow with the Flame enchantment and Power IV is a likely candidate for this. Once you enter the nether, a second source is bartering with piglins which requires large amounts of gold. Finally, you can of course brew potions, but brewing requires at least 2 blaze rods- you will have to attack a previous nether fortress before having access to this.