Anything can be bought from the US and Europe .In 1970-85 US intelligence successfully eliminated many WMD materials smugglers but as sophistication grew it became more difficult.
Reagan regime tacitly actually allowed Pakistan to develop its nukes program as Reagan regime saw Pakistan as an important ally in the Soviet Afghan War. What was seen as unimportant however became grave after 1990. Reagan administration had vastly underestimated the threat that would emerge in the entire region from North Korea to Iran.
By and large US policy to contain nuclear proliferation has been a failure and Iran war would be a watershed in this regard.