When reading through some unpacked files (for making my own mod) i found that G3Items.oc listed all Items of the game and put them in categories that may give a clue how to get them.
Here is what i found out about the "Special Artefacts" and "Event Items".
I have no idea of how the percentages are.

Please leave a comment below if this helped you or
if you have found out another way of getting an item!

OldBoot - Fishing "Failure" Item
HaresPaw - Hunting "Failure" Item

Rat - CatchRats (Children Activity from Home)
RatCompanion - Use Rat from Inventory of the Child

Mandrake - Gather from Home, by ALCHEMIST GUILD MEMBERS only
PotionOfFateWomen - Produces at Home, by ALCHEMIST GUILD MEMBERS only

Banquet - Produced at Mansion
CouncilorsChain - Produced at CItyHouse or Mansion
FamilySword - Produced at CItyHouse or Mansion
SignetRing - Produced at PoorHouse, CityHouse or Mansion
SlideRule - Produced at PoorHouse, CityHouse or Mansion
FlowerBouqet - Produced at CountryHouse, PoorHouse, CityHouse or Mansion
WashCloth - Produced at CountryHouse, PoorHouse, CityHouse or Mansion

Obligation - PlunderGraves, Guild Events
Purse - PlunderGraves, Fishing, Guild Events
Elegant Rapier - PlunderGraves, Smuggle goods
Remains - PlunderGraves (epic fail: you die and get this), Ingame Description says something about a kidnapped character dying too

Aquired by going on
Long Distance Journey or Trip Overseas:

• Amber
  
• Brocade
  
• Ivory
  
• Pearl
  
• Porcelain
  
• Purple

Far Trade:

• Salt
  
• Pepper
  
• Silk
  
• Tobacco
  
• Wine
  
• Purple

Note: Summoning can give all Far Trade and Journey Items too,
but might also hurt or even kill you!

ConstructionPlan - Freemansons
MetalHammer - Freemansons, Craftsmen

SignetGuildofMerchants - Merchants
DiamondStaff - Merchants

PoisonTrap- Alechmist
TalismanOfInspiration - Alchemist

SignetGuildofCraftsmen - Craftsmen

LockPicks- Thieves
BetrayedCaptain - Thieves

WarBanner - War (Officer)
WarHorse - War (Officer)
MedalOfHonor - War (Footsoldier)

This is my first guide and i just tried to share some of the knowledge i couldn't find otherwise.
Your Feedback is very welcome!