Since forced collectivization failed to increase agricultural yield, what means did Stalin have to fund rapid industrialization during the first five year plan.
By a combination of suppressing wages and letting peasants starve.
The problem of development is largely a problem of consolidation. Soviet Central Planners correctly concluded that the historic lesson of the Soviet Union's economic backwardness in the 1920s compared to the prosperity of their countries, especially the United States, lie in a lack of consolidation of resources and economies of scale.
Scale advantages are twofold in any major economy. First, a larger plant can produce more efficiently by more aggressively dividing labor. Second, a larger plant has more resources to invest in new technologies. Third and finally, larger industries can shut off their smallest and least efficient appendages and focus on the largest and most efficient units. Finally, larger industries can streamline production and focus on 1-2 products instead of the myriad of different competing products that would exist in smaller plants.
In order to achieve these scale advantages, huge amounts of capital had to be amassed. The Soviets fortunately had a huge agricultural base, and realized that it was mainly being diverted to consumption. Consumption is often regarded as a good thing in economies, and indeed it is in isolated economies. Without anyone buying goods, no industry could make profit. Fortunately for the Soviets, Europe in the 20s and 30s was not an isolated economy, and they could make money selling industrial goods to the rest of Europe. Domestic consumption was unnecessary.
Consequently, the Soviets embarked on huge confiscations to sell agricultural produce and gain foreign credit. The mechanism for this was collectivization, by which farms would be aggregated, giving the state control over the surplus, instead of letting peasants by their own goods. Collectivization also made it easier to introduce modern farm technology to larger, consolidated plots, which forced millions of peasants into the city, creating a cheap labor supply.
Mentioning cheap labor, the Soviets also saved money on growth by adopting the Group A/Group B theory. Group A goods were what Western economists called capital goods - those that could be re-invested, such as chemicals, steel, rail cars, and factory equipment. Group B were consumer goods, sold directly to the people and immediately consumed. The Soviets owned almost all industrial production, and concentrated on Group A goods, which they re-invested year after year to create more Group A goods, in an upward spiral of production.
The only thing they needed to pay for in these conditions was labor. Unlike a private capitalist in the West, who, when wanting to open a plant, needs to buy equipment and inputs, the Soviets created inputs specifically to invest in plants to create more of the same inputs. The influx of millions of peasants to the cities, meanwhile, increased the labor supply and reduced wages. Despite a massive expansion of industries and cities, average industrial wages in the USSR actually decreased by 17% in the first two five year plans. The central planners wanted to give ordinary workers just enough for housing and basic necessities, which greatly reduced the expense of industrialization.
The only true expenses the USSR had to incur were the basic investments for the first years of the plan which could not be covered by domestic production, and foreign expertise. The USSR paid high wages to Ford Motor experts, for example, to come to their country and teach them how to implement the assembly line. However, these expenses were start-up costs, and as Soviet industries expanded, they could in-source more and more responsibilities.
Sources:
Gronow, Jukka. Caviar with Champagne: Common luxury and the Ideals of the Good Life in Stalin's Russia.
Shpotov, Boris. The Ford Motor Company in the Soviet Union in the 1920s-1930s: Strategy, identity, performance, reception, adaptability.
Davies, R.W.. Years of Hunger: Soviet Agriculture, 1931–1933
Fitzpatrick, Sheila. Everyday Stalinism: Ordinary Life in Extraordinary Times.