Just playing through Konquest gets you enough.

^^^^^^^
What he said....I played through konquest...I didn't have to complete it to get enough coins...I probably had more than enough onyx coins by the time I made it into outworld, at the very least.
Also...try the coin trick from MK: DA
I am not sure if it works, i honestly haven't tried...but I have read on other forums that it works...but i have not confirmed that in any way.
If you're desperate for onyx coins though, it is worth a shot.
Noob-Smoke is a pretty fun character to use, the storyline and ending are stellar.

Buy the koffins that contain onyx coins