One of his main sources was the "Volsung Saga"
which you can read here.
This old Norse epic was also the inspiration for Wagner's "Der Ring des Nibelungen", which I often quote from in my chapter headings.
To cut a very long story short-
Alberich/Andvardi steals the Rhine gold and makes a magic ring from it.
Wotan/Odin asks the giants to build a castle, Valhalla for him.He steals Alberbirch's ring to pay them.
Alberich curses the ring to be desired by all and bring death to all who own it.
Wotan is now enthralled by the ring and refuses to part with it but relents when the Wala(pronounced Vala),Erda counsels him to relinquish it.
The curse takes immediate effect as one of the giants kills the other.
Many years pass and Wotan anxious to regain the ring, rathers a hero, Siegmund.Meanwhile, Alberich has also fathered a son,Hagen/Hogni while the surviving giant, Fafner/Fafnir has turned himself into a dragon to guard the Ring. Wotan's plans go awry and he orders his daughter Brunnhilde/Brynhild to see that Siegmund is killed in battle.
Brunnhilde decides to spare him,thereby incurring Wotan's wrath.He imprisons her on a mountain,where she will sleep surrounded by fire until a fearless hero awakens her.She will lose her immortality by marrying a mortal.
About twenty years later,Wotan has taken to wandering the world as an old man dressed in a grey cloak. Siegfried/Sigurd the son of Siegmund kills the dragon, Fafner and takes the magic ring for his own. He then awakens Brunnhilde and claims him as his bride.
Eager for new adventures, Siegfried leaves Brunnhilde after giving her the ring.He travels to the Hall of the Gibichungs when Gunter/Gunnar lives with his sister Gutrune/Gudrun and half brother Hagen.
They give him a drink to make him forget Brunnhilde and fall in love with Gutrune.Siegfried then agrees to win Brunnhilde for Gunther.
Betrayed and furious, Brunnhilde conspires with Hagen to kill Siegfried,which he does.
She then learns the truth and over come with remorse orders a funeral pyre to be built for Siegfried.Once it is lit she jumps on it and the flames rise to consume Valhalla and all the gods within.
The Rhine floods the hall and Hagen is drowned and the ring restored to the Rhine.
A new age of men dawns,now the gods are no more.
You can hear music from the Ring Cycle here.
and read the librettos here
I grew to know and love these operas when I was in my teens and the fact they were inspired by the same myths was what first drew me to Tolkien about ten years later.
There are some ideas found only in Wagner and Tolkien and not in the original myth,the most notable being the ring having the power to devastate the world and an immortal woman losing her immortality to marry a human.