Animated Mithril Armour is found at the Warriors' Guild. If you want to summon the monster, you need to place a Mithril full helmet, Mithril platebody and Mithril platelegs on the Magical Animator.
Although this can be a good way to get Warrior guild tokens, there is an opportunity that any piece of the armour could break during combat. Like all animated armour, the animated mithril armour is weak against Crush. Note that you will get your armour back most of the time when fighting.
This is a great way to achieve Warrior Tokens since the Animated Mithril Armour is comparatively simple to kill but still awards 50 tokens.