Minecraft wiki entry for Things_not_to_do, associated text: Trick a ghast or a blaze into shooting the portal again, to relight it. This does place the player in harm's way.
If the player has found a nether fortress, they can kill a ghast for gunpowder, blaze for blaze powder, and wither skeleton for coal, then craft a fire charge. However, all these mobs are dangerous.
Try to get lava to set wood next to the portal on fire so that fire appears inside the portal frame. Note that this does not work with crimson or warped wood.
Flint and steel, as well as fire charges can spawn in ruined portal chests.
Find a flint and steel in a nether fortress chest(or just iron, as flint can be obtained from gravel)
Bartering with piglins for a fire charge