The orginal source was tithing to begin with. I doubt the Church is bringing in so much tithing money that they can build hundreds of temples and run all church operations without expending earned interest. I am curious where you heard that only tithing money can be used. Maybe it is old information. The Church asked for donations to help rebuild the Nauvoo temple apart from tithing dollars. I remeber donating specifically to the Nauvoo temple fund. So in that case in there was money other than tithing used to build the temple. I am not old enough to know this for sure but I believe the LA temple and others of that geberation were also partially funded with donations apart from tithing money.