There are 2 ways that I know of to get enriched durasteel.
1. Belsavis daily area, any of the scavenging nodes has a chance of spawning enriched durasteel, its somewhat rare. You have to clear the normal nodes and hope for an enriched durasteel node to spawn.
2. Cartel packs give 10 of a prototype crafting mats, and enriched durasteel is one of the mats it can be.
3. "Rumor" has it that discovery investigation missions can give enriched durasteel if it crits?, but only heard it once and so far havent seen any proof of this.
3 is almost confirmed here, I've had some enriched durasteel in return of a mission but don't remember which one it was. Could even be TH...
4. Grade 6 pvp boxes, that's where I've had some the very first time.