His drapery shop

In his letters, Leeuwenhoek never made any reference to how he made a living, especially in the period before he began to work for the city of Delft in 1660.

We have two bills of sale to support the contention that van Leeuwenhoek sold all the materials for making clothes: fabric, especially linen, and affordances like buttons and accessories like ribbons.
In addition to this direct evidence, we have two other things. From ages 16 to 21, van Leeuwenhoek lived in Amsterdam and apprenticed with cloth merchant William Davidson. When he returned to Delft, he bought a house on the Hippolytusbuurt.