When Wickham arrived in London with Lydia, he went to Mrs. Younge for assistance. Mrs. Younge was Miss Darcy's governess at the time that Wickham had tried to convince Miss Darcy to elope with him. Mrs. Younge had tried to assist Wickham in that endeavor and was fired by Darcy as soon as he learned how she was involved. After being dismissed from service at Pemberley, Mrs. Younge bought a house in London on Edward Street and earned an income by letting out rooms to lodgers. Wickham went to her to try and get a room for himself and Lydia, but the house was full. Mrs. Younge advised him to lodge with a friend on a different street. Hence, Darcy was eventually able to bribe her enough to disclose Wickham and Lydia's location.
All of this can be found in Mrs. Gardiner's letter to Elizabeth explaining how Darcy became involved in
Wickham and Lydia's marriage (Ch. 10, Vol. 3).