THE BEST WAY TO GET THEM IS TO PLAY THE PUZZLE MODE ;) USE RAIDEN IF YOU HAVE HIM-BO RAI CHO IS OK TOO, BUT PERSONALL I'VE WON MORE MATCHES WITH RAIDEN ;) SCORPION IS GOOD TOO IF YOURE PLAYING WITH OTHER HUMANS ;)

here are some koffins that give you onyx koins:
TF - 434 koins
JG - 357 koins
RH - 397 koins
NH - 461 koins
NA - 476 onyx
use em wisely

I played Puzzle Kombat over & over w/Bo' Rai Cho & Kabal, they seem to work the best in my opinion, though I haven't tried Raiden yet. Sub-Zero works well. Baraka sucks in PK, horribly. So yeah, good luck! And on another note, if you're like me, you'll have problems gettin gold koins. The solution is simple: Chess Kombat.
Sucks that I hafta start over. *sigh* Stupid Max Drive.....