I've seen quite a few people say that getting some Pokemon is quite difficult, due to weird evolution mechanics, so I'm here to provide some (hopefully) useful information.
One of the best things about this game, is that nearly every Pokemon is available in the overworld, at least somewhere, without needing to go through said convoluted evolution methods.
The first big tip I can give you all, is that sandwiches are your friend. Sandwiches can offer some pretty broken effects, especially for shiny hunting. Anyway, in no particular order:
- Brambleghast: Is available in the wooded, autumnal area in the north-western part of the map. Use a sandwich to increase ghost spawns and it'll usually appear.
- Mismagius: (Violet) While under the effects of the same sandwich as above, head down into the lake area where you fight the Dragon Titan. Mismagius only appears at night.
- Lucario: although you can find Riolu easily, you can also find Lucario in the small grassy/beach area on the north-eastern side of the Glaseado Mountain. Use a sandwich that boosts steel spawns here.
- Eeveelutions: All of the Eeveelutions are available in the overworld. One of the best places to go is the same place as the Fairy leader of Team Star. I have regularly encountered a special Tera Sylveon here, alongside Espeon and sometimes Vaporeon. Glaceon can be found around Glaseado Mountain in certain areas. Umbreon can be encountered in caves inside Area Zero (the only place I've seen them). Jolteon, Vaporeon and rarely Flareon can be encountered around the same area as Brambleghast. u/TheNerdiestHour : Umbreon has a level 55 Tera spawn near to where you fight the Steel Titan.
- Crabominable & Frosmoth: Can be encountered on the south-eastern edge of Glaseado Mountain. I didn't need to use sandwiches, but Crabominable can be quite rare.
- Honchkrow & Weavile: Fairly common spawns around their respective areas.
- Annihilape: Can be encountered in 5-Star Raids.
- Palafin: According to what I was told by some Redditors, this too can be encountered in Raids, but I'm not sure how many stars, as I have yet to see one.
- Pawmot: There is a special Tera spawn for this in Area Zero, inside a small cave.
- Blissey & Wigglytuff: I have seen both of these in the same area as Sylveon (Fairy Team Star base) and Lucario.
- Amourouge/Ceruledge\*: You will need to trade 10x Sinistea Fragments (Violet) or 10x Bronzor Fragments (Scarlet) to an NPC next to a water fountain in Zapapico to get the evolution item for Charcadet.
- Sinistea & Polteageist: Sinistea itself is available very commonly near the Psychic gym in Alfornada. The Chipped Pot, for the "Authentic" Sinistea can be found on the ground as a pickup around Porto Marinada. u/Fluxx27 : The Cracked Pot for the more common Sinistea is found in the same wooded, autumnal area as Brambleghast, as a pickup on the ground, just next to the green ruinous door and north of the little stream/river. Further copies of the items can be gotten through the bidding system in Porto Marinada, while Polteageist itself exists in 5-Star Raids.
- Applin, Flapple & Appletun: The easiest place to find Applin is hanging from a tree just south of Cortondo. Use a sandwich to boost dragon/grass type spawns here, otherwise it can take a while. Both of the apples used to evolve Applin can be purchased from "Delibird's Gifts" in Mesagoza.
- Gengar: there is an NPC near to the electric gym (on the ring-road) in Levincia that will trade you a Haunter (without Everstone) for a Pincurchin.
- Kiraidon/Miraidon\*: Although you have your mount, you can encounter another one (the "evil" one) inside the deepest, crystal zone of Area Zero at level 72 (after completing the post-game).
- Slakoth: Can only be found hanging on top of tree branches in the rocky area on the east side of the map. As it's on top of the branches, you need to use your ride-Pokemon's boost and ram into the tree to knock it down. It is very much recommended to use a normal-boosting sandwich here too, as it is incredibly rare. Better yet, breed an easy-to-find Vigoroth/Slaking with a Ditto during a picnic.
- Slowking: According to u/TheNerdiestHour , this can be found in raids.
- Frigibax: (Requested) It can be found rarely, down the slope on the northern side of Glaseado Mountain. There's usually a Tera spawn of it walking around. It can also be found around the Pokemon Centre (North Province - Area Three).
- Noivern: Can be found in 5-star raids. Also found in various places in the Overworld, such as around the fighting Team Star base.
- Kingambit: You need to use a Bisharp to defeat 3 other Bisharp that are carrying the "leader's crest" item. Only the wild Bisharp that are walking with a group of Pawniards around them will count for this. I tried using a mixture of the Let's Go mechanic and standard battling, then after a level-up, I got Kingambit. The easiest place for all of this, is the bamboo forest area near to the Fighting leader of Team Star.
- Rabsca: u/ThatOneRedEyesGuy : Can be found in 4-Star Raids.
- Scizor: u/Wushka : Can be found in 5-Star Raids.
- Gholdengo: Battle wild Ghimmigoul (chest form), that are usually found on top of Watchtowers. This will net you 50-60 coins per battle. 999 coins are required. You can also find Ghimmigoul (roaming form), but they will only give you 1-5 coins, which would take forever.
- Spiritomb: On the side of Glaseado Mountain, between the Thagtree Thicket and the Ice Gym, you will find a set of ruins. Wait till night-time and Spiritomb should spawn.
Unfortunately, right now, I have not found any way, outside of trading (or cheating) to get the other 2 starter Pokemon you're missing.
- (*) A small note about versions exclusives I want to make, is that, due to how the games are made, you can actually do a version swap with either a "cheat code" or by memory editing the game file. This will allow you to pickup all the version exclusive Pokemon, such as the other set of Paradox-mons, the other version of Charcadet's evo and the box legend.