Pakistan has two options outside of US suppliers if that is an issue for an agreement for an upgrade. The Ozgur from the Turks or a Leonardo Upgrade by the Italians.
But realistically if they approve an upgrade they are gonna want the deal for US companies.
The F-16V upgrade is the same the Taiwanese will get and the US may not want the PAF to have the same sub-systems integrated into networks/datalinks with Chinese equipment operated by the PAF, potentially revealing some capabilities.
Looking back at an old post of mine from 3 1/2 years ago, the global used F-16 inventories that could be plausible source of air frames would be:
USAF active and retired
Various US state Air National Guards
30 Danish F-16AM
60 Dutch F-16AM
46 Norwegian F-16A
43 Belgian F-16A
30 Portuguese F-16
200+ Turkish F-16
63 Jordanian F-16
Unlikely the Turks will sell when they have their own needs. Jordan looks like the best option. Greece would be a long shot, hence why I did not include them.