You can change the skin on any item you have to any skin you have unlocked, so what you saw was a Legendary Staff (either Bifrost, Nevermore or Aurene's Insight) with the Funerary Staff skin applied to it.
You can get many recipes of the TP (like Yassith's Spire) and from drops around the world, in general if you don't know how to get something you can search thee wiki, either by going to the webiste or by typing "/wiki yourSearchTerm" into chat to open the wiki search.
For many of the Heart of Thorns stat combinations (like Viper's which you want to craft) it is easier to craft a core Ascended item and then change the stats via the mystic forge because you can avoid crafting Fulgurite this way.