By time you go through Konquest once you should of had enough to unlock everything onyx. If not just go into puzzle and do a few matches to get you some.

If you want more onyx coins just train with Noob-Smoke again in Chaosrealm.

yeah this happend to me... even after i beat konquest...i just played like 3 puzzle matches and i had enough to get what i needed

When I was done Konquest and finished unlocking everything, Onyx were the koins I had the least of, but I still had thousands of them after unlocking every tomb.

Heh, I always had plenty of onyx coins, I was very short of gold coins though, and that was because of my own stupidity. I was playing really late one night(like 5am) and had maybe 3000 gold coins, so I decided to buy some stuff from the krypt. I was so tired I didnt even realize I was buying coffines I had already purchased, by the time I realized I had spent around 2500 of my gold....
Oh and to answer your question, puzzle kombat is your best bet.

find meat in the netherrealm at 10 - 11 pm in the cave with the skeleton and he will help ya out :D

Play Puzzle and win, you get tons of onyx koins

you can also go defeat Kobrea/Kabal/Kira in netherealm to get an additional 1,000 onyx coins.
There was someone else in Netherealm that gave me a ton of Onyx coins for giving them something. They were in the town along wth wall. I think they gave me 600-1000 onyx coins.
But yeah, I had a major Onyx coins shortage even after beating Konquest and I searched every bit of every map.

thanks for the imput guys, this should help out.