I'm yet another player who spends a considerable amount of time hunting familiars and monster cards (762/752 monster cards, 310 familiars, currently level 259 in Reboot).
Right now the only way to know which monsters drop which items to look through the crusader codex. Monster cards that you've collected in the codex will display, among other useful information, the drops for those monsters. For certain items with extremely low drop rates such as the Daiwa Sword from Leader A, world boss familiars like Lyka, and instanced boss familiars like Lord Skeleton, this is the only way to verify the existence of their drops with absolute certainty.
Will we see a new source of truth for drops to replace the codex?
Secondly, in JMS there are many more monsters that drop familiars. Currently in GMS it is impossible to obtain familiars from Arcane River through drops. Familiars that do not drop from monsters MUST be obtained through the cash shop and are only obtainable on non-Reboot servers.
Will all the monsters that drop familiars in JMS also drop familiars in GMS?