You can short on Kraken. Fair warning: Kraken is absolutely not suitable for time sensitive trading. Their back end cannot handle low volume days, let alone high volume days. You can expect to enter the same order 5-10 times before it goes through without unspecified failures, and this is challenging because it may not accurately report whether an order succeeded at the API level, or was actually filled. But if your strategy can tolerate some uncertainly in when you'll be able to make an order, I guess it's ok.

You can short ETH on some trading websites such us eToro or Plus500 which in my opinion are not really great. A really high risk move would be to register on 1broker which is a CDF broker where deposits are done in BTC. There you can short BTC if you like. I would surely not leave any amount in that broker for a long time

Another way of shorting is to get an ETH loan from someone, covert it to $ and then wait for ETH to fall. When it does you can buy more ETH, pay back the loan and keep the profit.

If you use coinbase then you should try to login to GDAX with same credentials. It just works and the fees are like 0.25% per trade.

P.S.: GDAX is owned by Coinbase

Bitfinex is probably your best bet.