Just a quick written guide on where to find all the non-standard classes in one singular page. Old guides are based on the Vita/3DS releases; the later Steam release added drop rates to the EXP mobs (the Sagittas family of monsters) to make it easier to farm them. This guide includes those locations as sourced from personal experience and various fandom wikis.

Any unmentioned classes can be made through normal Classmating, though the Heroine may need to be a higher level, higher bond, or you may need to combine with Tri-Mating for higher stat caps overall. Tri-Mating is unlocked via hitting certain minimum bond with multiple girls, the level of which cannot be reached until at least Chapter 4.

Bondsman:[conception.fandom.com]

• All non-item, base classes born at least once (do not all have to be in your reserve)
  (Swordis, Thief, Grappler, Archer, Magician, Cleric, Merchant, Gunslinger, Paladin, Magic Knight, Ranger, Berserker, Lancer, Hunter, Blacksmith, Gambler, Minstrel, Ether Sniper)
  
• Combine with Dungeon Master to make enemies into Matryoshkas by level 49

Astromancer:[conception.fandom.com]

• Chapter 5 (Wrath Labyrinth unlocked)
  
• Attacker+healer combination class at high levels

Warlock:[conception.fandom.com]

• Priest + Magician to level 25 in your reserve at the same time
  
• Super magic damage dealer

Dungeon Master:[conception.fandom.com]

• Clear 5 purple sub-labyrinths (chapter 6+)
  
• Dungeon utility skills, such as seeing traps and fleeing labyrinths
  
• Combine with Bondsman to make enemies into Matryoshkas by level 49

Bulleteer:[conception.fandom.com]

• Ether Sniper + Gun Saint (item required) to level 30 in your reserve at the same time
  
• Very stronk, much wow

These items are one use and you must farm as many of them as you want of the children for that class.

Monsters can be stolen from for a higher chance for the item, but they can still drop from being defeated regularly. With specific repeatable Research Lab quests, the Sagitta-type enemies can be marked on the field to make finding them easier. When looking for non-Sagitta groups, remember that the MC's Dusk Analysis skill will allow you to see what each monster symbol is composed of in the overworld.

Team compositions such as Thief/Thief/Thief or Thief/Merchant/Dungeon Master will give higher passive drop rates by level 10, and Thief/Trickster/Ninja will give you a special full team stealing skill by level 11. The drop rates can be very low, so don't be discouraged. Keep trying!

Mercenary [Mercenary Manual]:[conception.fandom.com]
- Swordis upgrade
- ♂ Male only

Witch ♀ [Magical Girl Heart]: [conception.fandom.com]
- Magician upgrade
- ♀ Female only

Diva [Chanteuse's Dream]:[conception.fandom.com]
- Minstrel upgrade
- ♀ Female only

Gun Saint [Golden Will]:[conception.fandom.com]
- Ether Sniper upgrade, required to unlock Bulleteer
- ♂ Male only

Dark Knight [Fallen Knighthood]:[conception.fandom.com]
- Paladin upgrade

Ninja [Hermit's Gleam]:[conception.fandom.com]
- Thief upgrade, combine with Thief/Trickster for a strong stealing team skill

Trickster [Vibrant Sparkle]:[conception.fandom.com]
- Thief upgrade, combine with Thief/Ninja for a strong stealing team skill