Medium armor kits can be crafted by Leatherworkers with a skill level of 100. The pattern is learned from a leatherworking trainer NPC.
Medium armor kits are sometimes also found as drops, particularly from chests.
You can also receive medium armor kits as rewards from the following quests:
Armor kits fill the same virtual "slot" as enchantments and will overwrite any existing enchantments, or be overwritten by enchantments applied later.
Stacks in lots of 10.