I now have a Playstation coders cable on sale.
Simple design, works under all operating systems. Works with the PSXSERIAL code upload tool, which can be downloaded here http://www.psxdev.net/forum/viewtopic.php?f=60&t=378. Also works with Shendo’s BIOS Dumper and PS1 cardlink. Compatible with all models other than the PSOne (it doesn’t have an external serial port).
Drivers for the cable are available from here http://www.silabs.com/products/mcu/pages/usbtouartbridgevcpdrivers.aspx
Only £12.50 + Shipping and will ship worldwide. (Europe typically around £3-4) + Add 5% PayPal fees if paying via PayPal
Update 01/01/2018: I still have these cables for sale and are shipping pretty much worldwide. Email me at email@example.com with any enquiries