You can use Blocktrades as @timcliff mentioned. If you are interested in alternative, you can buy Steem from cryptocurrency exchanges (Binance, Bittrex, Poloniex etc.) and send it to your Steem wallet here. Then simply power up. Steem/Eth pair is available on Binance. However, you can sell Eth for Bitcoin and then buy Steem from Steem/Btc market as well.

Steemit account wallet is your Steem wallet and @chidimicheal is your wallet address (Steem username).