Buy bitcoin first on an exchange site and then buy NEM at a trading site like Poloniex or Bittrex. That's what I did with my NEM holdings.
What exchange do you suggest to buy bitcoin from other than coinbase?
Try Bitfinex or Paxful, but there are a lot of exchanges depending on which country you're from and can google some too whichever you're comfortable with because there are some whose fees are high.