All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the questioner means by the phrase 'a place where men and women mix' markets, streets and the like, then we do not know any objection or harm in collecting funds from such places, if the person who collects the money is keeping with lowering the gaze and repelling the harm. Since it is permissible for a Muslim to go to such places to fulfill his needs and interests, then it is also permissible for him to collect funds for building a mosque. So we do not see any objection to this and we do not say that such collected funds are forbidden funds. However, if the questioner means by mixed places, places like bars, bistros and cinemas, and other places where there are other prohibitions than just mixing, then it is not permissible in principle to go to such places for collecting funds, or anything else. Nevertheless, it is permissible to use the funds that had been collected from such places for building the mosque.
Finally, for the ruling regarding residing in non-Muslim countries, please refer to Fatwa: 86405.
Allaah knows best.