In my experiences killing Bandits for Mithril Armor is a ton easier than finding the shop to buy it. Typically shops won't store all pieces of the armor and they won't have it until you reach higher levels. Nothing seems to come to mind for places that have Mithril armor floating about so my only suggestion is that if you want to reach a high level fast just rubber band your controller while doing an action that corresponds to a Major Skill. If you find any higher level armor at a low level then your extremely lucky. But in my 3 complete play-throughs in which I got almost every single item in the game (no joke, the only ones I couldn't get were a few quest items, but I got almost all the other quest items), I can't remember a time I found any set of high level armor at a level significantly lower than the requirement. Just enchant the highest armor you have with a Shield Spell or get Grey Aegis and you should be set for defense.