There's binance and poloniex. I just use bittrex though. Has worked fine for me. Another thing you can do is just use the internal market at https://steemitwallet.com/market and exchange it for STEEM. There 0 fee exchange rate in the market, so you can exchange as often as you want. Once you have STEEM, you can power up or try exchanging STEEM at bittrex.

Again, bittrex is what I mainly use and has worked fine for me. What's the problem that you are having?