This is the only circumstance when I have found PI's - yet to find any in any kind of USS so far, but who knows what the next may bring?Doesn't have to be signal sources - I've got them from Deadly/Elite mission target enemies that interdict on trade/courier missions.
I have been aware that's the most efficient, but fragging condas is much more interesting...I promise you the best way to get these is to go to Imperial central space around their capital system and pick up imperial shielding by the absolute bucketload, then go trade.
You can always try the correct method which is finding HGEs in Outbreak aflicted systems, but it's extremely hit n miss even if you find an hge in the right state, it is rare that it will contain pharmaceutical isolators. The criteria you want in order to give you a decent chance, is simply an independent OR alliance system (to avoid pollution by superpower specific mats), that has at least one faction in a state of outbreak. Scan the nav beacon, check for HGEs, and check them all even if they are not in a state of 'outbreak'! The label on the USS seems to have little effect on the state the mats you find SHOULD relate to. For example, even in a 'none' state uss, if there's an outbreak faction in the system, you might get pharma isos. That's it.
PERsaonlly, I use the imp shielding method, FD will surely fix this soon, it's so crap.