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.