They are $58 at PBTECH!
for my usb null modem, straight off amazon. insane prices locally.
Any comments made are personal opinion and do not reflect directly on the position my current or past employers may have.
According to this, an "HP Procurve" serial cable is DB9 at one end and RJ45 at the other.
If I was you, I'd get a DB9 connector from Jaycar, get a bog-standard Ethernet cable and chop it in half, and wire them up as above.