Kensington K64406us Washable Usb/ps2 Keyboard - Usb, Ps/2 - 104 Keys - White - English (us) : Target
Amazon.com: GLHONG PS/2 to at Keyboard Cable Adapter - 6 Pin Mini Din Male to 5 Pin MIDI Female Keyboard Adaptor : Musical Instruments
Here's a prototype keyboard adapter for the PS1. It was intended to be used with the planned modem. Sony did create an official version without the cord, but was never released. :
USB to PS/2 Adapter - Keyboard and Mouse - USB Adapters (USB 2.0) | Cables | StarTech.com
PortaSound PS-1 - Keyboard Instruments & Music Production Tools - Display collection - INNOVATION ROAD - Yamaha Corporation
Original IBM PC AT Keyboard, Thoroughly Cleaned, original unmodified layout – Brand New Model F Keyboards
Compact USB Keyboard (PS/2 Protocol) - for uMSX from Denjhang's Retro Hardware on Tindie