Craft the Fire and Ice Bows
First you must find the Blacksmith's Recipe for each bow, then craft it in the Blacksmith's in Bree. They cost 8 Mithril Bricks each.
Mithril Fire Bow
This can only be attained in Freeplay on the mission "Helm's Deep".You'll need either the Berserker character or the Mithril Fireworks to get this one. Once the wall is breached, climb it and use Gimli to destroy the barricade to the left. The recipe is inside a Mithril box just behind it.
Mithril Ice Bow
This can be found on the mission "Cirith Ungol". As you're pushing a pillar with one of the characters past windows with wooden boards on them, make sure to stop at the first one. Behind it is the recipe.