DC7900 USDT Power Supply

Hi,

I have an old HP DC7900 USDT PC that I wish to give to my children so that they can get into computing.  However, I lack a power supply.  It uses the old external brick power supply and runs at 135W.  Does anyone know where I can get an HP original PSU from?  I have looked in the usual places, but I am unable to find a HP original - as my kids will be using it I do not necessarily trust 'other' providers.

Thanks in advance,

Matt.