Component blueprints usually drop from bosses. Rhino's drop from the boss of Venus, Oberon's from the Earth boss, etc. Every time you complete the mission you get a random one; it took me five tries to get all three for Rhino, and thirteen tries to get everything for Oberon.
I looked it up and it seems Nekros BPs drop from Orokin Derelict Assassination missions. To reach the Derelict you have to build keys, using the Nav Coordinates you've no doubt been finding, among other things. You'll likely need to recruit help for those missions (Derelict missions require you to invite people if you want help). I'd say it's doable, but you're right in that it could take a while.