Here are some very appropriate saints to whom I pray for chastity:
St. Augustine of Hippo (struggled with sensuality and fornication himself)
St. Thomas Aquinas (tempted to lust but preserved his chastity perfectly)
I am a member of the Angelic Warfare Confraternity, a fellowship of men and women dedicated to pursuing and promoting chastity together under the powerful patronage St. Thomas Aquinas and the Blessed Virgin Mary.
Here is the Prayer of St. Thomas Aquinas for Purity which is part of our daily prayers in the Confraternity:
Dearest Jesus! I know well that every perfect gift, and above all others that of chastity, depends upon the most powerful assistance of Thy Providence, and that without Thee a creature can do nothing. Therefore, I pray Thee to defend, with Thy grace, chastity and purity in my soul as well as in my body. And if I have ever received through my senses any impression that could stain my chastity and purity, do Thou, Who art the Supreme Lord of all my powers, take it from me, that I may with an immaculate heart advance in Thy love and service, offering myself chaste all the days of my life on the most pure altar of Thy Divinity. Amen.
St. Mary Magdalene would also be a good intercessor.
And best of all is Our Lady, the Holy Mother of God, the Spotless Virgin Mary.
"Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you."