Thanks for suggestions. You confirmed that poloniex is a way to go as I hoped.
Poloniex is the largest altcoin trading platform now. The volume there for Ethereum is huge. It is more than all the other exchanges combined.
I've read from somewhere here in the forum that there are some kind of hack-related issues on certain accounts in PoloniEX. Just to be sure, spread your assets into 2-3 exchanges so if something comes up you're not fully hit.