im no sure but check the avikan furnace or metal working station

You can find Violated Droden and Drahl in random encounters on desert planets. Both of those enemies have a chance to drop Thelean materials, so that's the best place to go hunting!

Additionally, the Drifter's Emporium - a black market vendor in the Starfarer's Refuge - also sells Thelean materials, in addition to a lot of other quest items.