2 Added easy-to-digest summary at top of answer.
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TL;DR

Allowed sources are

  • Written sources found in the course of adventuring
  • Spellbooks/Book of Shadows/ritual books of wizards/tomelocks/ritual casters in your party.

"Finding" means finding a written copy of a spell. (Tweet from JeremyECrawford)

frizbog to mikemearls, JeremyECrawford: For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?

JeremyECrawford to frizbog, mikemearls: Finding one written down.

For example, a tomelock cannot inscribe a prepared spell. (Tweet by Mike Mearls, knowledgeable expert, but not RAW.)

SorcererQzot to JeremyECrawford, frizbog, mikemearls: Can they write down a spell they have prepared already?

mikemearls to SorcererQzot, JeremyECrawford, frizbog: no - requires creation of a scroll or spellbook

One written source is other players' spellbooks. (Tweet from JeremyECrawford)

LordHighMarshal to JeremyECrawford, frizbog, mikemearls How would one get a ritual spell if it is in another party member's spell book?

JeremyECrawford to LordHighMarshal, frizbog, mikemearls The warlock could copy the ritual spell from that spellbook.

Can rituals be copied from another warlock's Book of Shadows or from a ritual book? I've seen no ruling which explicitly says these are permitted. However, since "finding" means "finding a written source" and since using another player's wizard spellbook is specifically permitted, copying from another Book of Shadows or ritual book fits the same definition of "finding" as copying from a wizard's spellbook.

"Finding" means finding a written copy of a spell. (Tweet from JeremyECrawford)

frizbog to mikemearls, JeremyECrawford: For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?

JeremyECrawford to frizbog, mikemearls: Finding one written down.

For example, a tomelock cannot inscribe a prepared spell. (Tweet by Mike Mearls, knowledgeable expert, but not RAW.)

SorcererQzot to JeremyECrawford, frizbog, mikemearls: Can they write down a spell they have prepared already?

mikemearls to SorcererQzot, JeremyECrawford, frizbog: no - requires creation of a scroll or spellbook

One written source is other players' spellbooks. (Tweet from JeremyECrawford)

LordHighMarshal to JeremyECrawford, frizbog, mikemearls How would one get a ritual spell if it is in another party member's spell book?

JeremyECrawford to LordHighMarshal, frizbog, mikemearls The warlock could copy the ritual spell from that spellbook.

Can rituals be copied from another warlock's Book of Shadows or from a ritual book? I've seen no ruling which explicitly says these are permitted. However, since "finding" means "finding a written source" and since using another player's wizard spellbook is specifically permitted, copying from another Book of Shadows or ritual book fits the same definition of "finding" as copying from a wizard's spellbook.

TL;DR

Allowed sources are

  • Written sources found in the course of adventuring
  • Spellbooks/Book of Shadows/ritual books of wizards/tomelocks/ritual casters in your party.

"Finding" means finding a written copy of a spell. (Tweet from JeremyECrawford)

frizbog to mikemearls, JeremyECrawford: For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?

JeremyECrawford to frizbog, mikemearls: Finding one written down.

For example, a tomelock cannot inscribe a prepared spell. (Tweet by Mike Mearls, knowledgeable expert, but not RAW.)

SorcererQzot to JeremyECrawford, frizbog, mikemearls: Can they write down a spell they have prepared already?

mikemearls to SorcererQzot, JeremyECrawford, frizbog: no - requires creation of a scroll or spellbook

One written source is other players' spellbooks. (Tweet from JeremyECrawford)

LordHighMarshal to JeremyECrawford, frizbog, mikemearls How would one get a ritual spell if it is in another party member's spell book?

JeremyECrawford to LordHighMarshal, frizbog, mikemearls The warlock could copy the ritual spell from that spellbook.

Can rituals be copied from another warlock's Book of Shadows or from a ritual book? I've seen no ruling which explicitly says these are permitted. However, since "finding" means "finding a written source" and since using another player's wizard spellbook is specifically permitted, copying from another Book of Shadows or ritual book fits the same definition of "finding" as copying from a wizard's spellbook.

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source | link

"Finding" means finding a written copy of a spell. (Tweet from JeremyECrawford)

frizbog to mikemearls, JeremyECrawford: For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?

JeremyECrawford to frizbog, mikemearls: Finding one written down.

For example, a tomelock cannot inscribe a prepared spell. (Tweet by Mike Mearls, knowledgeable expert, but not RAW.)

SorcererQzot to JeremyECrawford, frizbog, mikemearls: Can they write down a spell they have prepared already?

mikemearls to SorcererQzot, JeremyECrawford, frizbog: no - requires creation of a scroll or spellbook

One written source is other players' spellbooks. (Tweet from JeremyECrawford)

LordHighMarshal to JeremyECrawford, frizbog, mikemearls How would one get a ritual spell if it is in another party member's spell book?

JeremyECrawford to LordHighMarshal, frizbog, mikemearls The warlock could copy the ritual spell from that spellbook.

Can rituals be copied from another warlock's Book of Shadows or from a ritual book? I've seen no ruling which explicitly says these are permitted. However, since "finding" means "finding a written source" and since using another player's wizard spellbook is specifically permitted, copying from another Book of Shadows or ritual book fits the same definition of "finding" as copying from a wizard's spellbook.