Warframe: the Quick and Dirty Guide to Acquiring Warframes

• Ash: Market (BP), Venus Proxima (Systems), Neptune Proxima (Neuroptics), Pluto Proxima (Chassis)
• Atlas: The Jordas Precept/Simaris (BP), Jordas Golem Assassination (all parts)
• Banshee: Tenno Lab (all parts)
• Baruuk: Little Duck (all parts)
• Caliban: Market (BP), Narmer Bounties rotation A (Systems), Narmer Bounties rotation B (Chassis), Narmer Bounties rotation C (Neuroptics)
• Chroma: The New Strange, Uranus Junction (Neuroptics), Neptune Junction (Chassis), Pluto Junction (Systems) OR Cephalon Simaris (all parts)
• Citrine: Tyana Pass, Mars (Mirror Defense - all parts) OR Otak's pity shop
• Dagath: Dagath's Hollow (all parts) - NB that she needs Vainthorns that are end of mission rewards for Abyssal Zone (keys are 5k Faction Syndicate Standing a pop)
• Dante: Armatus, Deimos (Disruption - all parts) OR Loid's pity shop
• Ember: Market (BP), Sargas Ruk Assassination (all parts)
• Equinox: Market (BP), Tyl Regor Assassination (all 8 parts)
• Excalibur: Market (BP), Lech Kril Assassination (all parts)
• Frost: Market (BP), Vor & Lech Kril Assassination (all parts)
• Gara: Saya's Vigil/Simaris (BP), Plains of Eidolon Bounties (all parts)
• Garuda: Vox Solaris/Simaris (BP), Orb Vallis Bounties (all parts)
• Gauss: Market (BP), Kappa, Sedna (Disruption - all parts)
• Grendel: Market (BP), mod-disabled missions (all parts, 25 Vitus Essence per key)
• Gyre: Zariman Bounties (all parts)
• Harrow: Chains of Harrow/Simaris (BP), Void Fissure-corrupted enemies (Chassis), Pago, Kuva Fortress (Spy - Neuroptics), Defection (any - Systems), Taveuni, Kuva Fortress (Survival, Systems and Neuroptics)
• Hildryn: Little Duck (BP), Exploiter Orb (all parts)
• Hydroid: Market (BP), Vay Hek Assassination (all parts)
• Inaros: Sands of Inaros (all parts) OR Simaris (all parts)
• Ivara: Spy missions (all parts)
• Jade: Jade Shadows (BP), Ascension (all parts) OR Ordis's pity shop
• Khora: normal Sanctuary Onslaught (all parts)
• Kullervo: Acrithis's shop for Kullervo's Bane (all parts)
• Lavos: Father's shop (all parts)
• Limbo: Market (BP), The Limbo Theorem/Simaris (all parts)
• Loki: Market (BP), Hyena Pack Assassination (all parts)
• Mag: Market (BP), The Sergeant Assassination (all parts)
• Mesa: Market (BP), Mutalist Alad V Assassination (all parts)
• Mirage: Market (BP), Hidden Messages (all parts)
• Nekros: Market (BP), Lephantis Assassination (all parts)
• Nezha: Tenno Lab (all parts)
• Nidus: The Glast Gambit/Simaris (BP), Oestrus, Eris (Infested Salvage - all parts)
• Nova: Market (BP), Raptors Assassination (all parts)
• Nyx: Market (BP), Phorid Manifestation (all parts)
• Oberon: Market (BP), Earth Proxima Points of Interest (Neuroptics and Systems), Saturn Proxima Points of Interest (Chassis)
• Octavia: Octavia's Anthem (BP, Mandachord), Lua music puzzle (Chassis), Terrorem, Deimos (Survival - Neuroptics), rotation A chace on Plato, Lua (Exterminate - Systems)
• Protea: The Deadlock Protocol/Simaris (BP), Granum Void (all parts)
• Qorvex: Whispers in the Walls (BP), Sanctum Anatomica Bounties (all parts) OR Bird 3's shop (all parts including the BP)
• Revenant: Mask of the Revenant/Simaris (BP), Plains of Eidolon Bounties (all parts)
• Rhino: Market (BP), Jackal Assassination (all parts)
• Saryn: Market (BP), Kela De Thaym Assassination (all parts)
• Sevagoth: Call of the Tempestarii/Simaris (BP), Void Storms (all parts)
• Styanax: Kahl's Garrison shop
• Titania: The Sliver Grove/Simaris (all parts)
• Trinity: Market (BP), Ambulas Assassination (all parts)
• Valkyr: Market (BP), Alad V Assassination (all parts)
• Vauban: Market (BP), Nightwave shop (all parts)
• Volt: Tenno Lab
• Voruna: Conjunction Survival OR Archimedean Yonta's pity shop (all parts)
• Wisp: Ropalolyst Assassination (all parts)
• Wukong: Tenno Lab
• Xaku: Heart of Deimos/Simaris (BP), Cambion Drift Bounties (all parts)
• Yareli: The Waverider/Simaris (BP), Ventkids' Bash Lab (all parts)
• Zephyr: Tenno Lab

Notes:

• 'Frames that aren't Assassination drops are bolded.
• Primes drop from Relics, and there's too many of those for me to list here. Ditto for "special" weapons (there are currently around 500 weapons in-game).
• Once you unlock the Circuit, you can choose the following 'frames as rewards for normal Circuit on a weekly rotation basis: Excalibur, Trinity, Ember; Loki, Mag, Rhino; Ash, Frost, Nyx; Saryn, Vauban, Nova; Nekros, Valkyr, Oberon; Hydroid, Mirage, Limbo; Mesa, Chroma, Atlas; Ivara, Inaros, Titania; Nidus, Octavia, Harrow; Gara, Khora, Revenant; Garuda, Baruuk, Hildryn.
• Some of those farms are decidedly easier than Assassination missions (Dojo research), but some of those aren't (Caliban with his time-gated BS springs to mind). Special mention to Equinox that needs 8 drops from its Assassination boss. IDK who thought that was a good idea.
• There are 17/52 Assassination farm 'frames.

The last Assassination farm 'frame to be added was Wisp and she was added to the game back in 2019. DE simply doesn't use this method any more.