There are 400+ exchanges supporting Ether trade and most of then have USD as an alternative option so you can check the real-time rate of ETH/USD pair on various exchanges (on a single page).
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https://coinmarketcap.com/currencies/ethereum/#native.
However, while considering the cheapest option also check the volume, reputation and hidden fees of the exchange to avoid the trouble. Since USD is not my native currency, I find that Paypal is one of the good options to hold USD (if received through exchange or organisation).