One day in the forest, a gandharva named Angaraparna attacked the Pandavas, but Arjuna incinerated Angaraparna's chariot with a fire-arrow and captured him.
Angaraparna's wife begged for mercy, and the Pandavas freed him. Angaraparna then rewarded the Pandavas with great hospitality, telling them many stories, and also giving them advice. "Find a priest, get a wife, and make yourselves the kings you were born to be."
So the Pandavas found a priest, and the priest told them to seek a wife at the swayamvara of King Drupada.
Because they were still hiding from the Kauravas, they went disguised as brahmins.
Inspired by: Jaya: An Illustrated Retelling of the Mahabharataby Devdutt Pattanaik.
Notes: This story is on p. 82. The Pandavas' priest is named Dhaumya.