What is a librería de viejo, and where are they found? Write your answer in English.

A "librería de viejo" is a second-hand bookstore that specializes in selling old or antique books. These stores can be found in various locations, including historic city centers, antique markets, and specialty neighborhoods known for their bookstores or vintage shops. They often feature a wide selection of rare and collectible books, as well as out-of-print and hard-to-find titles. Customers who enjoy browsing through shelves of dusty old books and searching for hidden treasures will appreciate visiting a librería de viejo.