Sold in Rome's four Coliseum Entrance zones for 1,000 ducats each from the Coliseum Clerk closest to the entrance/exit.
Also dropped liberally by one of the mobs in the carib land zones, e.g. Santo Domingo outskirts. If you go after shark reps for your company contributions, you're set to end up with a healthy stack of these on the side.
I found that the Cayenne shopkeeper likes these when I try to "donate item" with them in my inventory. For 200 somas, he gave 90 adv exp with wrong job type as well as 7 x
Patriot Award, without the
Contributing to your Country Oxford skill loaded. I guess it'd have been, what, 8 or 9 if I had remembered to load the skill.
Given that as per Guilder's post above, Colloseum clerks sell unlimited amounts of these for 1000 ducats each, it potentially looks like a decent way to farm PAs outside of EIB times, for anyone not scared to use their brains to think out of the box.
If anyone interested they can check their own colonial capitals to see if the shopkeepers there will accept them - I could only confirm Cayenne. Also, if anybody knows of any shopkeepers/peddlers in the Americas selling soma, that'd be cool, although I kinda doubt it (would be too easy!).