[Arran]

 Try fleabay for loctal sockets, you may have to settle for some Chinese repro ones though if you want new, other then that you may have to use salvaged ones from a junk chassis. Just so you know the dimensions on the Chinese sockets are often given in millimeters and not in inches, so make sure you do the conversions first before ordering. 
  Again, as Terry asked, why do you want to replace the sockets anyhow? Are they broken? I replaced all of the sockets in a five tube AC set about a year ago that used really crappy wafer sockets, those were octals, and whilst I have the technique down I won't say that I would do it again without good reason. It might be easier just to find a chassis from a 42-PT4 or 42-PT2 or 42-PT10 with a broken cabinet and good sockets rather then replace the ones in the PT7 since they all used the same chassis.
Regards
Arran