I will sort by generation:
RBY
In the Japanese version of Blue, you can trade a Kadabra for a Graveler (that evolves into Golem) on Cinnabar Island.
In the Japanese version of Blue, you can trade a Machoke for a Haunter (that evolves into Gengar) in Cerulean City.
In Yellow, you can trade a Marowak for a Machoke (which evolves into Machamp) in the Underground Path (Routes 5 and 6)
DPPt/HGSS
In D/P, you can catch wild Steelix in Victory Road, Iron Island, and Snow point Temple.
In Pt, you can catch wild Steelix in Victory Road and Iron Island.
In D/P/Pt, you can catch Gengar in the Old Chateau if you have any gen 3 game in the gba slot (Dual slot mode).
In HG/SS, you can catch wild Steelix in Cliff Cave and Mount Silver.
In HG/SS, you can trade any Pokemon with Jasmine at the Olivine Gym to receive a Steelix.
BW/B2W2
In B/B2, you can catch Huntail on Route 4 while fishing in rippling water.
In W/W2, you can catch Gorebyss on Route 4 while fishing in rippling water.
In B2/W2, you can trade Emolga for Gigalith (Route 7).
In B/W/B2/W2, you can catch Milotic (which evolves via trading while holding a prism scale in these games and onwards) on Route 1 while fishing in rippling water.
In B2/W2, you can catch wild Steelix in Clay Tunnel, Twist Mountain, and Underground ruins (Dust Cloud).
In B/W, you can catch Politoed in Route 6 while fishing in rippling water.
In B/W/B2/W2, you can catch Politoed in Routes 6, 19, and 20, Floccesy Ranch, Lostlorn Forest, and Victory Road (Cave Entrance) while fishing in rippling water.
In B/W/B2/W2, you can catch Kindra on routs 17 and 18, and P2 Laboratory while fishing in rippling water.
In B/W, you can catch Slowking in Abundant Shrine while surfing in rippling water.
In B2/W2, you can trade Hippowdawn for Alakazam in Accumula Town.
XY
In X, you can catch Huntail in Route 12 while fishing with a Super Rod.
In Y, you can catch Gorebyss in Route 12 while fishing with a Super Rod.
In X/Y, you can trade a Luvdisc in Cyllage City for a Steelix.
In X/Y, you can catch Politoed on Route 19 with a Super Rod.
In X/Y, you can catch Trevenant on Route 20.
SM/USM
In US/UM, you can catch Huntail and Gorebyss in Melemele sea and Route 15 while fishing and by sosing (Huntail at night, Gorebyss at day)
In S/M/US/UM, you can catch Politoed via sosing in Malie Garden at night while it is raining.
In US/UM, you can catch Slowking via sosing in Kala'e Bay.
In S/M, you can trade a Haunter for a Graveler (that evolves into Golem) in Tapu Village.
In S/M, You can encounter Conkeldurr via Island Scan on Poni Island during Mondays.
In S/M/US/UM, you can catch Gengar via sosing in the Abandoned Thrifty Megamart.
In S/M, you can get Trevenant in the poke pelago (after you reach Ula 'Ula island).
In US/UM, you can trade Phantump for a Phantump (which evolves into Trevenant) in Tapu Village.
SWSH
Alakazam can be found as a Wanderer in the Insular Sea and Loop Lagoon of the Isle of Armor
Machamp is a Wanderer in the Giant's Mirror and can be found in many Max Raid Dens
Gengar can be found as a Wanderer in the Giant's Cap and can also be found in many Max Raid Dens
Politoed can be found as a Wanderer in the Soothing Wetlands of the Isle of Armor
Slowking is obtainable in Dynamax Adventures in the Crown Tundra
Steelix is a Wanderer in Giant's Mirror and Giant's Seat in the Wild Area.
Scizor is a Wanderer in the Training Lowlands and Champion's Road in the Isle Of Armor
Kingdra is a Wanderer in the Honeycalm Sea of the Isle of Armor
Milotic is a Wanderer in the Lake of Outrage and South Lake Miloch in the Wild Area, as well as the Giant's Bed in the Crown Tundra
Rhyperior is a Wanderer in the Potbottom Desert of the Isle of Armor, as well as many Max Raid Dens in the Wild Area
Electivire is a Wanderer in the Giant's Bed, Frigid Sea, Three Point Pass, and Ballimere Lake of the Crown Tundra
Magmortar is a Wanderer in the Slippery Slope, Giant's Bed, Snowslide Slope, Frigid Sea, and Ballimere Lake of the Crown Tundra
Dusknoir is a Wanderer in the Stony Wilderness of the Wild Area
Gigalith is a Wanderer in the Dusty Bowl of the Wild Area
Conkeldurr is a Wanderer in the Motostoke Riverbank of the Wild Area, as well as the Giant's Bed and Giant's Foot of the Crown Tundra
Escavalier is a Wanderer in the Training Lowlands and Forest of Focus in the Isle of Armor, as well as many Max Raid Dens in the Wild Area in Sword
Accelgor is found in the same places as Escavalier, but can only be found in Wild Area raid dens in Shield
Aromatisse can be found in many Max Raid Dens in the Wild Area in Shield
Slurpuff can be found in many Max Raid Dens in the Wild Area in Sword
Trevenant is a Wanderer in the Slippery Slope, Snowslide Slope, and Old Cemetery of Crown Tundra, as well as many Max Raid Dens in the Wild Area
Gourgeist is a Wanderer in the Hammerlocke Hills of the Wild Area
BDSP
Steelix can be found as a wild encounter in Victory Road, Iron Island, and Snowpoint Temple
SV
You can trade Pincurchin for a Haunter in Levincia that evolves immediately. Gengar can also be found in 5 and 6 star Tera Raids as well as the Mossfell Confluence in Kitakami
Trevenant can be found in the Kitakami Wilds and Timeless Woods of Kitakami. It can also be found in 5 and 6 star Tera Raids
Golem, Milotic, Conkeldurr, Dusknoir and Politoed are found in 5 and 6 star Tera Raids in Kitakami
Scizor and Slowking are found in 5 and 6 star Tera Raids in Paldea
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Hope I Helped!
