This antenna has Arduino product code X000016. The product page says:
List of Compatible Boards
This penta-band antenna is supported by the following official Arduino boards:
So I read that as saying that it is not compatible with the MKR NB 1500. So it is not possible to purchase a spare antenna from Arduino for this board.
You should be able to find one from other sources though. The important part is to make sure that the antenna you purchase has the correct connector to attach to the board. This connector is most often known as "U.FL. What makes things a little confusing when shopping for antennas/pigtails/etc. for this connector is that "U.FL" is a trademark of Hirose Electric Group, so every other manufacturer who wanted to make these connectors had to call them something different. This means there are something like 6 different names for the same connector:
Hirose U.FL, I-PEX MHF series, IPAX, IPX, AMC and UMCC (Ultraminiature Coax Connector and Cable Assembly Style A Series from Tyco Electronics)
So you might find perfectly compatible antennas that just don't happen to refer to the connector as "U.FL".