Fossils are back in The Indigo Disk DLC for Pokemon Scarlet & Violet, but as already-resurrected Pokemon instead of items for you to breathe some life upon. Shieldon is one of them, and it’s out there available for catching for everyone who manages to find it.
Where to Find Shieldon in Pokemon Scarlet & Violet: The Indigo Disk
You can spot Shieldon in a secluded area in the Canyon Biome of the Terarium. Look out for the island with the heart-shaped lake and head there to find it. Walk for a bit around the zone and you should start seeing some Shieldon spawning here. It’s not exactly a rare spawn, but it might take a few seconds until you start seeing some of it.