Well, I don't have a comprehensive answer for you detailing all their sources of funding, and also the revolution itself was largely funded so to speak from the defection of Tsarist Russian assets and citizens in its active stage, but the Bolshevik party itself was at least partly funded through a series of bank robberies.

In the 1907 Tiflis bank robbery for example, Stalin was the Georgian local called on to organize the whole operation by the Bolshevik party center (and may have been directly involved in the robbery himself in person). The Bolsheviks chose this far flung region of the empire to ambush a large transfer of state funds by the Tsarist government. The trend seems to have been to conduct robberies in the extremities of the empire probably to make it easier to escape abroad with the money, such as in their 1906 Helsinki robbery in Finland. In Tiflis the robbers hit the money stage coach as it passed through the town square in a massive shootout with bombs and revolvers, and chasing the runaway carriage on horseback. Because there was a large police presence in the square, a large number of people were killed, ~50. They made off with several hundred thousand rubles, and Bolshevik party leaders including Lenin fled abroad in disguise to exchange and hide them. They used what money they could from this and other robberies mostly to buy weapons and support party activities in Russia.

This particular robbery came not long after a congress of Russian socialist revolutionaries in which the Bolsheviks had accepted the Menshevik's demand to cease militant fund raising such as bank robberies. In this way the Tiflis robbery sort of represented a further repudiation of the Menshevik's calls for less violent reform, and the increased division of the two factions.