If finding BPs via pol/mot is based on chance, then it is likely to take a long time to get a full set for any particular building, especially when duplicates are taken into consideration.
The same will apply to the BPs given as rewards for contributing to GBs; if they are random then there is a good chance of getting duplicates and not necessarily getting a full set.
I don't know if it would be possible to calculate the actual percentage, but would guess that it is pretty low. Therefore it is probably just useful to bear that in mind when looking for BPs - it will take a lot of work to get the Great Buildings, but if they were easy to get then they probably wouldn't class as great.... Good luck for finding them -I'm living in hope that I might get even one GB at some point!