First of all we obviously have Eevee itself. Fortunately, Professor Mirror has an Eevee of his own, meaning you can snap a few shots of this fan favourite as soon as you unlock the ability to take photos in your research camp shortly after the beginning of the game.
Vaporeon unfortunately can’t be snapped until after you’ve beaten the main story. This is because you can’t go to the location where it’s found - Maricopia Reef (Night) - until after the credits roll.
Once you’ve finished the game, make your way to the reef at night and head to Machamp’s rock, which Primarina will now be sitting on. Once you pass this, you’ll get an environmental scanning prompt. Interacting with it will inform you that there’s a Pokemon under the waves - press R to play music and Vaporeon will jump up into the air before diving back down into the water. If you catch it mid-jump, you’ll get a handy four-star snap.
Flareon is also a late game Pokemon, not being encounterable until you get Fireflow Volcano to rank three. Once you’ve done this, it’s all over the place. It’s sleeping in the end location of the left route from the junction, and it appears twice on the right route - once next to Charmander in the blue and grey room, and again wandering around the inside of the volcano at the alternate end location. It takes a while to make your way to a level where Flareon actually appears, but once you get there, it’s everywhere.
Umbreon is arguably the most difficult Eeveelution to find. It’s in the last area of the main story, Ruins of Remembrance, and you’d never know it was there unless you spent quite a while trying to figure it out.
Once you get to rank three, head through the spawn area and down through the crater where Houndoom is asleep. Immediately turn left and chuck an Illumina orb at the Noivern so it moves. Once it does, look down the corridor it was facing and spam everything - fluffruits, music, Illumina orbs… I’ve had success with all of them. Once you get its attention, Umbreon will come out and run right past you. You can encounter it again later in the map when it’s asleep next to the fourth Crystabloom pillar. Wake it up with an Illumina orb and watch what happens… Honestly, it’s worth the arbitrary, almost silly summoning prerequisites.
Espeon can be found in Elsewhere Forest once you get it to rank three. While this is difficult, it is literally the first or second Pokemon that appears on screen once you load up rank three, so… yeah, enough said.
Glaceon appears in Shiver Snowfields at rank three. After going through the intro area and out onto the open plains where several Mamoswine roam around, you’ll go down a steep hill towards a cavern. Right before this, look to the mountain on top of the cavern and you’ll see Glaceon. If you snap it, it will appear again near the end of the level, in the inaccessible icy area behind where Spheal and Piplup are.
Leafeon appears in rank three of Founja Jungle. It’s basically the first Pokemon you see and spends the entire trip walking just that little bit too far ahead of you. It veers off into the last cave on the left before the teleportation point, but aside from that you can see it most of the way through the level. Unfortunately, I still haven’t cracked how to get it to like me...
Sylveon appears in Florio Nature Park (Night) at rank three. In the middle section, where all of the Torterra are sleeping, keep your eyes peeled for the last Torterra in the line. It’s sleeping where Tangrowth appears during the day, at the far side of the lake but before you make a turn towards the Taillow sign. Wake it up with an Illumina orb and it will move to sleep next to the last Torterra in the level instead, which is located in the meadow that leads to the teleportation point. You can get a fancy four-star snap here if you play your cards right.
Jolteon can be found in Outaway Cave. Once you hit rank three and can choose which way to go, take the right route from the junction. After going through the tunnel with fighting Geodude on your left and a sleeping Croagunk on your right, you'll see two Carbink and a Mawile on the ground. Throw Illumina orbs at all of them and Diancie will appear. If you throw another orb at Diancie, it will emit a blast of light that calms down the other Mawile a little further in. Now that it's not angry anymore, it will allow you to pass, opening up an alternate route. You'll go past some crystals and enter an open area filled with plants - Jolteon is immediately on your left, hiding behind some bushes.
I hope I've helped!