What does Della do to get money for Jim's present?

Initially, Della scrimps and saves pennies from their weekly shopping budget and is only able to collect $1.87. Dismayed that this is not enough for a proper Christmas gift for Jim, she turns to other ideas.
She goes to a shop run by a Madame Sofronie: "Hair Articles of All Kinds." Della asks Madame Sofronie if she would be willing to buy Della's long, beautiful brown hair. Madame Sofronie agrees and pays her $20. Della is then able to use this $20 to buy Jim a Christmas gift, namely a gold watch chain for his prized pocket watch.
Selling her hair for $20 is a great personal sacrifice for Della. The author makes clear that the couple is experiencing financial hardship, and does not own many nice things. However:

The James Dillingham Youngs were very proud of two things which they owned. One thing was Jim’s gold watch. It had once belonged to his father. And, long ago, it had belonged to his father’s father. The other thing was Della’s hair.

Della sells not only something intimately hers, but also an object of worth and pride in an otherwise impoverished home.