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There's two diffrent ways to get augments on 50+ items.

ANNMs drop random equipment with Augments on them.

During the mission for Crystals, you receive Key Item Keys that can be used at the treasure chest in the Tenshodo and you receive random items with aguments on them. You can also repeat the final mission as many time as you want (considering you do the climb to get the key item everytime) to keep getting new items.