Argthtjtr

In my D&D 5e campaign, the characters fought and defeated a young dragon of mixed white and silver lineage, and found a ring of mind shielding on her body. Her original body was destroyed. Unbeknownst to them, her soul has been preserved in the ring and hungers for a way to find a new body.

She is now conversing on a daily basis with the new wearer of the ring and is trying to influence him towards a way that would enable her to achieve corporeal form again. She would prefer a dragon body, but she is also used to assuming humanoid form (from her silver lineage), so that would be acceptable.

What magic documented in any of the official rule books could enable her to find a new physical form? Spells, magic items, or magical phenomenon such as a pool of radiance are all acceptable answers.

If the body was destroyed, the dragon's only real options are Wish and True Resurrection. All other revival and resurrection spells require a body.

The dragon can theoretically manipulate the player wielding the ring, but that's not really acquiring a new body.

Maybe (a retroactively cast) Magic Jar and Glyph of Warding?

The Magic Jar spell (PHB, p. 257) enables a creature to send its soul into a prepared "gem, crystal, reliquary, or some
other ornamental container worth at least 500 gp": there's nothing in the text indicating it couldn't be a magical ring. While in that object, their soul can subsequently attempt to possess a humanoid who is within 100 feet of the container.

The spell has quite a few details, and you should study it to make sure you understand all its ins and outs (for example, if the container is destroyed, even while the soul is possessing a new body as a feature of this spell, the caster [i.e. dragon] will die). But the most problematic part of the spell's description for your particular case is the fact that it has a 1 minute casting time.

If this dragon was killed in combat, it wouldn't have had time to concentrate on this spell for a full minute in order to cast it. But it might have been able to set up a Glyph of Warding (PHB, p. 245-246) a long time ago to prime the Magic Jar spell with a Spell Glyph. This could have served as a last ditch "escape plan" when it knew it was definitely going to die. Glyph of Warding's spells activate without having to undergo a longer casting time, so the spell could be set off in the midst of combat.

The only problem about this plan is that when you cast Magic Jar, your body becomes catatonic. The dragon would have needed to activate the spell glyph (by doing some pre-arranged trigger for the glyph, possibly something that costs a Reaction, like casting the Shield spell) just a few moments before it died, so that no one in the party would realize this was happening. If you have a party with members who have very high Passive Perception bonuses, it may strain believability that no one noticed the dragon fall down insensitive before the final blow was actually struck. But if this seems believable to you, it's a viable way that the dragon could cheat death (for the moment anyway: again, read the Magic Jar spell carefully to understand its limitations).