The service for purchasing STEEM and STEEM Power that @sthumacher mentioned is provided through Steemit by @blocktrades. You can access it with the button on your wallet page.

Other options with more steps include buying on a liquid market like Bittrex or Poloniex. Then you send the STEEM to your account from those markets and power up on your wallet page.

You can buy a limited amount of STEEM Power with a credit card using @Changelly but be aware that due to the risks of credit card purchases, they charge a premium compared to Bitcoin value times STEEM market value (traded as a BTC pair). Currently that premium looks to be about 24% over market.