Evidently, my one opportunity the find one went bye bye when I sold the tombs contents. Not knowing a quest in a mod addon would need it

Why not just add it through the console?

If you have Dragonborn, there are clams that have pearls at Soltsheim. Other than that, it's a rare item on alchemy shops and enemies that have alchemy ingredients. I think the only guaranteed one is Geirmund's Hall

some consider adding items through Console as "cheating"..


as for the OPs situation:

If you have the "Dragonborn" DLC, you can get them quite easily in Soltheim, just search the coast for "Pearl Oysters"..

if you don't have Dragonborn, they sometimes drop from Forsworn or Warlock enemies.. Peddlers you encounter on the road somtimes have them... and they will occasionally be in Apothecary Satchels...

Probably a Console Player. Either way, I think there's some Clams at the Bottom of Lake Ilinalta in the Falkreath Hold and the Sea of Ghosts

Those clams don't drop pearls. Only the oysters on Solstheim do.

If you have Hearthfire and especially build your house with a kitchen, you will get small pearls, I think there will be one or two downstairs in the cellar as well if you build all the shrines. If you do not mod the Hearthire homes on the PC to prevent them respawning everything, they will respawn when the house resets in 10 days so your pearl problem will be solved. Apart from that I remember the sodding things being pretty rare in the vanilla game, I got some in some tomb and I do not think they respawned, some others were in some Dwemer ruin, the one in Markarth I think or it could have been Alftand, but do not remember them respawning either, and i hated going down there. Do not remember seeing any of the Skyrim traders with them, but you could always try to buy some, maybe they changed with all the patches and DLC. I never had enough to even find the alchemy effects till Dawnguard turned up, before that satchels were the best source, so always check them.

If you only have Dawnguard installed and have finished it Serana's mum will have a few pearls in her collection, and I think in the Forgotten Vale those Falmer liked to collect pearls as well, but you will have to look for them, empty the contents of some baskets, check every hut. And as already mentioned, we got lots of small pearls with the Dragonborn DLC, where we could finally harvest our own. And those oysters I am pretty sure do respawn.

So they gave us plenty of pearls with the DLC, just not in the original game.

But surely the mod that needed the small pearl also added one some where?

IMO this was definitely an oversight by Beth.

There should have been a way to harvest them vanilla game... failure.