How many followers from a foreign religion does your city need to have for you to purchase their stuff? In this game I got the 3rd religion, I think, and all the buildings where gone by then. Later on I captured 2 missionaries from a religion that has Mosques which would benefit me a lot since I have tons of wine but, when I expended them on my cities I didn't have the option to buy Mosques? I'm almost certain I've been able to buy foreign stuff before so I don't know what's going on there... Anyone know?
The other civ's religion has to be the majority religion in a city where you want to buy that religion's religious buildings (i.e., the religion symbol on the city banner has to change to that religion).