Where to buy AVAX?
Check this FAQ on how to send AVAX from Binance/OKex to MetaMask
And on the Zero Exchange of course! On the Avalanche chain.
If you don't have AVAX to trade on the Avalanche chain (AVAX is used to pay for the gas fee, like ETH on the Ethereum chain) and you have never used the bridge on the Zero Exchange, you could buy ZERO tokens on the Ethereum or BSC chains. Then when you cross chain the tokens to the Avalanche chain, you will receive a fraction of AVAX to pay for the transaction fees, that you can use to swap ZERO for more AVAX.