Credit card and bank transfer options are fairly limited. We'd suggest you buy some ETH or BTC through Coinbase and then exchange it for NKN on either Binance or Bittrex.
Bittrex
Bittrex is a US based exchange founded in 2013 and headquartered in Seattle Washington. The company was founded by Bill Shihara, Richie Lai and Rami Kawach, all three previously worked at Microsoft.
A 0.25% flat commission is applied to all trades. Trading fees are slightly higher on Bittrex compared to the industry average. Additionally, there are neither rebates for higher-volume trades nor incentive models for takers and makers.
Binance
Binanace is the leading crypto to crypto exchange around with over 100 coins supported for deposit and withdrawal.
The exchange accepts users from around the world and has received favorable reviews from the crypto community. Recently the exchange has also partnered with Simplex to allow credit card purchases of cryptocurrencies at a premium.
A flat rate of 0.1% is very lucrative fee for any trader who doesn’t need immediate fiat liquidation. Incredibly, it undercuts most other large exchanges, some of which charge upward of 0.2%. That’s a pretty sizeable difference if you’re working with large orders.