Where to buy a replacement charging lead for my Beats Solo Pro?

a quick google suggest it's just a normal ligthing cable

Amazon.com: Beats Solo Pro Wireless Noise Cancelling On-Ear-Kopfhörer - Apple H1 Kopfhörerchip, Class 1 Bluetooth, 22 Stunden Hörzeit, integriertes Mikrofon - Schwarz : Elektronik & Foto

so one of these 2 would work, which one to get depend on if you wish to connect it to usb-a or usb-c

Lightning to USB Cable (1 m) - Apple

or

USB-C to Lightning Cable (1 m) - Apple

This is a user forum, you are not addressing apple here just fellow users

you can try contacting apple Support ->  Get Support (apple.com)

mac computers don't have lightning port connectors

some would have usb-c and usb-a but not lightning port

this is how they look (lightning port and usb-c) https://ichef.bbci.co.uk/news/640/cpsprodpb/411E/production/_110607661_usb_cables640_v2-nc.png

and this is what an usb-a connector looks https://www.maxgaming.dk/bilder/artiklar/16254.jpg?m=1588925664