My source is any novel/non-fiction or newspaper article I would be reading when encountering the word. I usually copy the sentence, maybe trim it a bit, and cloze on the word. The hint I give depends on the word, sometimes an English translation, sometimes a definition in the language itself, or sometimes a cognate word in a different language I know.

An additional benefit is that I remember the content of books more easily, since I am passively reminded of passages through Anki. This means I can put aside a book for months and get back into it without problems.

A problem, though, is that because I only use active recall of the word, I sometimes can't remember the meaning of the word when I encounter it, especially when the context is different. This can be quite subtle. E.g. I might put in "aborder" (to approach) in the context of "how would you _approach_ this question", but then when I read somewhere "the man was approaching" I would recognize the word, but be unable to make sense of it.

I have been trying to remedy this by sometimes choosing a more typical example sentence (from a dictionary or something) rather than the encountered one, which could be too poetic. But as I remarked, with all these changes it is hard to measure the effect in the long term.