You can use the Codex on your Orbiter to see all the available mods and where they drop - or you can search the Warframe Wiki. The Void Trader Baro Ki'teer will also sell some random Primed or dual-stat mods when he visists a relay station. If you have unlocked Lua there are also some special Drift mods to be acquired by completing the random Halls of Ascension challenge rooms. Riven mods can be rewarded from the daily Sortie, some dual-stat mods can only be aquired from Nightmare Mode missions, Orokin Derelict missions, Sortie (Riven mods). Mods drops from enemies when running your missions however some mods only drop on special missions e.g. Syndicate mods) may only be compatible with a specific Warframe. Most Warframe mods may be used on all warframes but others (e.g. Sniper Rifle), Shotgun, Pistol and melee mods. Mods are usually divided into Warframe, Rifle (incl. In the beginning you only have a few weak mods but as you progress through the Star Chart you will acquire more mods including some more powerful mods. Mods are used to improve the stats of your weapons and Warframes.

How to get prime mods warframe