Difficulty sourcing a 6mm dc adapter/plug/converter...any ideas?
The first thing to do is check if your plug has 2 or 3 pins. Most high current laptop plugs use the outside of the cylindrical sleeve for -ve and the inside of the same sleeve for +ve (it looks from your picture that this plug might be one of them). The centre pin is then usually used for IR compensation, or authentication (if you're unlucky enough to have DRM in your PSU).

Most of these connectors are expected to be soldered onto a cable and then have a PVC over- moulded cover and strain-relief applied. So they're not really much use if you want to do it yourself.

Your best bet it to find a connector pre-attached to a length of cable and then join it to your new PSU.

aliexpress, and to some extent ebay, have plenty of hits for "6mm laptop plug".

When you join it either use a premade cable joiner (pretty large and ugly), or solder and heat-shrink. If you use glue lined heatshrink for the outside it should gain some strength.

Bear in mind a laptop may not survive having their input power shorted out, so be careful making your cable. (and obviously check it matches the polarity of the original)