Conversation so far:
What are picts of Celtic art?
The Picts were a confederation of Celtic language-speaking peoples.
Were the picts of Celtic art in Scotland?
The Picts shared modern Scotland with a zone of Irish cultural influence on the west coast, including Iona, and the Anglo-Saxon kingdom of Northumbria to the south.
What other art did the picts of Celtic art produce?
Insular styles heavily influenced Pictish art, with interlace prominent in both metalwork and stones.
Are there any significant works by the picts of Celtic art?
The St Ninian's Isle Treasure of silver penannular brooches, bowls and other items comes from off the coast of Pictland and is often regarded as mostly of Pictish manufacture.
Any other notable Celtic art pieces besides The St Ninian's Isle Treasure of silver penannular brooches, bowls and other items comes from off the coast of Pictland and is often regarded as mostly of Pictish manufacture by the picts?
Pictish stones
Are Pictish stones in Scotland?
Yes

Question: Where else?
Self-contained rewrite: Where else are the Celtic art Pictish stones found, besides Scotland?