History seems full of poor artists who weren't famous until after their deaths. If the artists were penniless and not able to sell paintings, how did they afford paints and canvas? Even in modern times painting seems a hobby you need money for. Were paints just easier to get? Did they make their own? It seems like it would be a lot harder to have a wide color selection and good brushes and canvas and everything.
Van Gogh painted a few portraits (including this one) of Julien Tanguy, the guy he bought his art supplies from. Note that he "often accepted paintings in exchange for the goods he sold." Van Gogh's letters to his brother are riddled with references to Tanguy, for example this note that "I saw Tanguy yesterday and he put a canvas I had just done in his window."
Van Gogh and Tanguy did have some quarrels over missed bills and money, and it seems that Tanguy's wife was less than happy with her husband's willingness to sell supplies on credit. But Tanguy loved art and artists, and his goodwill helped get Van Gogh the supplies he needed when he needed them.
caveat that my area of expertise is the Dutch and to a lesser extent the English art market
Pre-mixed paint sold in tubes only came about in the early 19th century, prior to which point paint had to be mixed from oil or another binding solution (egg whites, gum arabic), and pigments (which could range from everyday substances like charcoal to precious stones like lapis, which would be purchased, usually from the equivalent of a modern-day chemist, at exorbitant prices). This would usually be done by an apprentice or specialised member of the artist's atelier.
The atelier system meant that casual or hobbyist art-making, or the Romantic concept of a single starving artist in a candlelit garret, didn't exist as we know it today. Hopeful artists would work as assistants to established ones, or enrolled in the established academies, building networks with dealers, commissioners, patrons, and guilds. Many artists were born into artistic dynasties, cf. the Bruegel family and the Teniers family and people like Rachel Ruysch. The supply vs demand balance in the art market was different - at one point, around 2/3 of homes in cities like Delft contained artwork, meaning that the economic engine was able to support even less established artists.
Vermeer is a special case - as I can recall, there is no conclusive record of under whom he studied. His output was quite low compared to many of his contemporaries, and while he used some of the most expensive pigments available, he didn't die particularly wealthy. Vermeer's family owned an inn, however, so he had another source of income with which he could finance his art-making.
Tl;dr poor artists are a relatively recent invention
Further reading/sources: it cuts a wide swath but I love the Janson's Art History Bible. Simon Schama's The Embarrassment of Riches has good information about the Dutch art culture. A Strange Business: Making Art and Money in Nineteenth Century Britain is a good look at the patronage system. Additionally, Tim's Vermeer provides a good picture of eighteenth century methodologies - pigment making and so forth, and delves into the Vermeer backstory as well.
Sources that I unfortunately don't believe are available in English: I have a book of the letters of Peter Paul Rubens which shed a lot of light on the atelier system. Additionally, Vlaamse wandtapijten in 50 verhalen by Koenraad Brosens, while specifically about tapestries, goes into the insane networks that artists and artist-adjacent merchants and craftsmen had throughout history.