You can buy them or make them.
Most app IIs are buyable from NPC vendors (called trainers). App IIIs and IVs are player made. Adept Is are dropped (from chests), adept IIs don't exist (except for your training upgrades at lvls 10, 20, 30, 40, and 50), and adept IIIs are player made, but from rare materials. Master Is are dropped from chests, but are quite rare.
You won't need to worry about adept IIIs or masters for a while; app IVs and adept Is should be plenty.