I have found this prayer to the Holy Souls in the book,"Stories About Purgatory," 30 Days For The Holy Souls.
A PRAYER ADDRESSED TO THE POOR SOULS IN PURGATORY:
O Holy Souls in Purgatory, thou art the certain heirs of Heaven. Thou art most dear to Jesus, as the trophies of His Precious Blood, and to Mary, Mother of Mercy. Obtain for me, through thine intercession, the grace to lead a holy life, to die a happy death, and to attain tothe blessedness of eternity in Heaven. Dear suffering souls, who long to be delivered in order to praise and glorify God in Heaven, by thine unfailing pity, help me in the needs which distress me at this time, particularly (here mention your request), so that I may obtain relief and assistance from God.
In gratitude for this intercession, I offer to God on thy behalf the satisfactory merits of my prayers, works, joys, and sufferings of this day (week, month, or whatever space of time you wish to designate). AMEN
