I am sure I recently saw some packages which contained gold nuggets.

John said:

You get gold nuggets in the Coffers of the Gods which (very occasionally) drop when yielding level 9 Gifts of the Gods tiles awarded to a Kingdom for placing first in a Fury event.

Forgot to mention I was level 29 palace. I've been able to get enough bronze through Ubers and tasks to upgrade about 5-6 of the smaller buildings to level 29, but it'll take forever to get enough for the market, oracle, etc. without having to buy packs, which I don't want to do anymore. I built up what I thought was a fortune in bronze from all the Ubers I killed waaaaaayy before I went for level 27. Not so.

You need bronze for levels 27-31. Well, technically level 26. You're first expense is upgrading the palace to 27. You don't need it again after everything is upgraded to lvl 31.

There's a great place on this forum that shows exactly how much everything will cost (food, wood, bronze, gold nuggets, etc) for every level of everything in the game.

You may have seen it but Plarium has posted an article about Special Items (including gold nuggets and bronze) on the pc login page

https://plarium.com/en/strategy-games/vikings-war-of-clans/news/special-items/

Bronze is an item necessary for upgrading buildings between levels 27 and 31. It can be earned by completing personal Tasks, or purchased in the Clan Store for Loyalty Points. Moreover, you can knock Bronze out of Uber Invaders. The chance of getting depends on Hero's Skills, learned Knowledge, equipment, and boosts increasing the Hero's combat stats when fighting Invaders.

A Gold Nugget is an item for upgrading buildings between levels 32 and 35. It can be obtained as a reward for passing checkpoints in Competitions and for taking top places in the Asgard League rankings.

All the items mentioned above, except the Warrior's Draft and Soul Shard, can be purchased as part of Bank offers.

Yes, I assumed Palace level 29 also and I haven't been that far so I don't know how much Bronze we're talking about but it's obviously doable considering the many towns at that level or higher. But I don't know which of the 4 ways is most cost effective given that there are other benefits than Bronze in each case.

As for the Palace level for Bronze and Manuscripts from Personal Tasks, it's definitely after 15 so somewhere between 16 and 20 then.