3 questions: use of “un”, algunas vs. algunos, and use of en vs. de

1) Why do you say “No tienen casa” and not “No tienen un casa”?   2) Is it “algunas” and not algunos because it agrees with personas?  3) Could you also say, La gente buscó ayuda de iglesias… as well as en iglesias?   This is the first time I did one of these exercises and I found it really helpful!