To come by something is to obtain it, often by luck. It arrived and you did not have to hunt for it.
It nearly always appears in a comment about difficulty, since saying that something is hard to come by means it is scarce, and that is the commonest use of the phrase by a long way. It takes a direct object. There is a second and quite separate sense meaning to visit briefly, so asking somebody to come by later is an invitation.
Practised in A Man Called Ove, A Song of Ice and Fire
First appears in chapter 7 of A Man Called Ove.
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