I actually was just about to register on bittrex. When I tried a message popped up saying that they halted new member registrations, something about such a large number of people registering all at once that overwhelmed them or something. Weird.
Try Binance or Poloneix, they also have many available top altcoin that in coinmarketcap. You can also try coinexchange and cryptopia for other alternative exchange or if you want to arbitrage.