Getting investment money together is the start of a process called capital formation. Investment money is the initial capital. The source of investment money is savings. A large number of consumers do not spend all of their money in the present. They save part of it.
Saving is postponed consumption.
Instead of spending today to consume now, some people save in order to be able to consume later. Money set aside in savings earns interest. Both the interest and the original investment can be used for consumption in the future. The money available for investment, especially for loans to business, comes from the savings of all economic units, individuals and organizations. It may be a very large amount, but it is a fairly stable amount.