If you follow the historical timeline, it's pretty easy to see where money had to have come from. I posted a more detailed timeline below, but the upshot is that the Church has been in financial difficulties a number of times, including as recently as 1959 (the last year the Church published financial statements). Each time, the Church turned to tithing to either keep the Church afloat, secure gap loans, or finance investments - and prior to the Church's policy change in 1959, much of this was publicized.

Even in 1991, President Hinckley claimed that the income generated by Church investments was dwarfed by tithing revenues ("We have a few income-producing business properties, but the return from these would keep the Church going only for a very brief time. Tithing is the Lord’s law of finance. There is no other financial law like it.")

A lot can change in 30 years, and it's possible that since 1991 the Church's investments have matured to a level that enables the Church to be self-sufficient without tithing funds. But if President Hinckley was telling the truth in 1991 (i.e. if investment revenues were still small relative to tithing revenues in 1991) and given what we know about the Church's finances up to 1959, where do you think the early investment money came from? Even if the investment money came from loans, what do you think the Church pledged as security for the loans?