Here is a popular prayer to ask for help in locating a lost object:
Saint Anthony, perfect imitator of Jesus, who received from God the special power of restoring lost things, grant that I may find (mention your petition) which has been lost. As least restore to me peace and tranquility of mind, the loss of which has afflicted me even more than my material loss.To this favor I ask another of you: that I may always remain in possession of the true good that is God. Let me rather lose all things than lose God, my supreme good. Let me never suffer the loss of my greatest treasure, eternal life with God. Amen.
Why is St. Anthony invoked to help find lost objects? Well, this can be traced back to a story about the saint and his beloved book of psalms. In Anthony's day, long before the invention of the printing press, all books were composed by hand and, therefore, were very, very expensive. Anthony had a book of psalms which he used in his daily prayers. He was living in a community of Franciscans at the time and one of the newer members became disenchanted and decided to leave the order. As he left the community, he took Anthony's psalm book with him. When Anthony realized that his book was missing, he prayed fervently that it would be found. Simultaneously, the thief had a vision of Anthony and felt compelled to return the book. He also decided to return to the Franciscan Order.
Incidentally, at a California parish which bears his name, an important relic of St. Anthony was stolen early yesterday. The parish priest put the relic (a small piece of the saint's body) out for the faithful to venerate, as yesterday marked the 780th anniversary of the saint's death. Unfortunately, the relic was stolen before the morning Mass began. Police are investigating. You can read a news story on the incident here. "St. Anthony, St. Anthony, please come around. Something is lost and cannot be found..." Perhaps there is hope for the relics return and for repentance for the thief.