I don't think I've ever seen it said that you cannot work for non muslims, except in that it's preferable to work for muslims, especially if non muslim bosses infringe on your obligatory islamic practices.
People often write to shayook asking for advice on how to deal with bosses who won't let them pray, or won't let them wear hijab
. I can't remember ever seeing a shaykh advising the person that working for a non muslim is haram.
From the hijab advice, check this out:
The Prophet (peace and blessings be upon him) said, “There is no obedience to anyone in disobedience to Allah.” In other words, you are not bound to obey your boss if he orders you to disobey Allah. In Islam, we are not even allowed to obey our parents if they order us to disobey Allah. So you don’t need to apologize for wearing hijab.
A layperson may take that hadith and say that it means that we can't have non muslim bosses. However, the scholar, who knows more about the hadith then what is presented at face value, uses it in a way to mean that we can have non muslim bosses as long as they're not making us disobedient to Allah (swt).
Here is some advice
given to someone worried about working for a jewish company. Notice that there is nothing in the answer that says it's haram to work for a non muslim, but instead addresses the problem of supporting israel.