@marynaw For improved Chickasaw plums both Odom and Guthrie are worth trying to get. Also @coolmantoole knows a lot about Chickasaw plums/hybrids and might be able to sell you some or give you good advice.
Chickasaw Plum Inventory - General Fruit Growing - Growing Fruit
Prunus angustifolia ‘Odom’ Plum 1 gallon - Mail Order Natives
Prunus angustifolia ‘Guthrie’ Plum 1 gallon - Mail Order Natives