old imac as main monitor or external monitor

The demo use the imac 2009 model and one cable. The cable is display port to mini display port. If you want to connect from macbook pro to old imac, you can buy a cable usb type c to mini display port. Where to buy the cable ? Reference link: (for reference only) 1/ DisplayPort to Mini DisplayPort Mini DisplayPort (Thunderbolt) to DisplayPort cable (Mini DP to DP) – 3m – (UHD with 4K / 60Hz, Version 1.2, for PC & Mac, DP male connector with locking feature, black) – TOP Series 2/ USB Type-C Male to Mini DisplayPort 6ft USB Type-C Male to Mini Displayport Cable 4K@60hz USB C to Mini DP Video Convert Cable Support Screen Mirroring Mode & Extend Mode 3/ VGA to Mini DisplayPort Multibao ThunderBolt Mini Display Port DP to VGA HD Cable Female Lead Adapter for Apple Mac Macbook