The example sentence in Why is も used instead of が in the sentence 「日本ではクモを見ると良いことがあると言う人もいますよ」,
got me thinking "Why is it using で, not に?". I.e., why not
It seems obvious that 日本では is intended to modify います. If it were modifying 言う, the meaning would be roughly "There are people who - when in Japan - say that ...".
Although I would personally use に in this sentence, it doesn't sound completely ungrammatical to me as it is. So why is that, given that いる usually takes に, not で?