To get the Mythril you have to go to the Semitt falls which is west of Salamand. When your on floor B5 there is a door that leads to the Mythril. It is guarded by a soldier who is not very hard. Use magic on him to finish him of quickly. When you have the Mythril go to Altea and talk to the blacksmith in the weapon shop and show him the Mythril. The item not the keyword. You will them be able to buy Mythril weapons and armour in Altea. You don't have to do the Mythril quest but it is useful.