From Aleteia:
It’s not too easy to see, but the official canonization portrait of Mother Teresa depicts what’s known as the “unity cross” on her rosary.
The unity cross is linked to the Schoenstatt movement, as it was created by the first generation of Chilean Schoenstatt seminarians who studied in Brazil and Switzerland.
The unity cross depicts Jesus on a low cross, enabling Mary to be pressed against him, as she collects his blood in a chalice which is at the center of the image.
According to Schoenstatt, the unity cross was crafted by Fr. Angel Vicente Cerró, when he was a seminarian. It was finished in 1960 and presented at the Schoenstatt Shrine in Bellavista, Santiago, Chile.
Its significance:
Monsignor Peter Wolf explains that “Mother Teresa found [a replica of] this cross in the dirt on the street and knew nothing of its history. She discovered it again when she visited the Vatican and saw that it was the pectoral cross of Archbishop [Francisco] Errázuriz [of Santiago, Chile], who was then the Secretary of the Congregation for Consecrated Life. Mother Teresa told him that this cross best expressed what she and her community wanted to do: Like Mary to stand by the Cross of the Dying Christ and to meet him there. The Archbishop, later Cardinal, then gave her hundreds of copies of this Unity Cross for her Sisters.”
The deacon, as minister of the chalice during Mass, shares a special bond to the Precious Blood—and in that bond, a closeness to the suffering of the Body of Christ.
Check the link for more information on the history of the cross. And a quick Google search turns up several places that sell the cross online, including Amazon!