RPG is not that difficult to obtain.It’s available from the Mujahideen era in Afghanistan and also it’s variants are being used in Iraq,Chechnya and other places(The reason I cited these two places because of their regional importance). Moreover,I would say that RPG or such weapons are not that `sophisticated’ these days compared to the standards of fight hardened people like Taliban ,or erstwhile L.T.T.E and others.Even Hamaz produces R.P.G 's domestically,like yasin, Al Bana etc.Ofcourse for an organization like Taliban,who knows in and out of Afghanistan it would be easy for them to produce them indigenously…(Just like they are making I.E.D’s)
I would invite your attenion to the fact that Kalashnikoves and other weapons were openly available in Peshawar .(I do not forget the fact that many of them might have been smuggled from outside). So there is no wonder that TTP is having it’s indigenous supply of weapons.