"Be Mine!" can be obtained from Imp and Cherub item vendors found within Aphrodite's Realm, as well as console merchants found in the Watchtower and the Hall of Doom during the 2011 Valentine's Day Event. These items can be purchased for 1 Aphrodite's Token each.
Obtained from Edit
"Be Mine!" is one of six treat consumables that must be used in order to earn the feat I Choo-Choo-Choose You!. This feat is unique to the event and is required to earn the metafeat A Hallmark of Success.