For any Mac made in the last 10–12 years, you typically do not buy a physical "piece" of software.
: Highly sought after as the final version to support PowerPC Macs. buy osx
: You can source official components for newer models through the Apple Self Service Repair Store. Current Software Delivery For any Mac made in the last 10–12
If you meant a specific accessory or replacement part to go along with your Mac purchase: : buy osx
: Apple and Amazon sell official USB-C and MagSafe 3 chargers (starting around $39–$79 ).
Physical retail "pieces" of the operating system are mostly used for restoring older PowerPC or early Intel Macs.