It can come from various sources. The Party asks for contributions from all members, and a part of the collective funds can be directed towards supporting a few professional revolutionaries. There were other ways the Party could gather funds for that, even bank robbing like Stalin famously did. But a Party can sell stuff, organize raffles, etc., I see this as the main basis behind supporting professional revolutionaries.

Lenin had a few "gigs" to earn money. After he received his law diploma, he practiced law in Samara Regional Court for a while, but he also gave private lessons to people. He advertised this work in newspapers at the time.

In principle, a professional revolutionary doesn't need money at all. They only need a place to live and food. Party members could have spare rooms in their houses which any professional revolutionary could stay for a while, meanwhile their food is guaranteed by any peasant, cook, etc. from the party willing to share and prepare food. You could have professional revolutionaries solely dependent on class solidarity, money doesn't necessarily have to be a concern when your party is organized enough.