You got very few exchanges to choose from. Exchanges that offer USD/USDT pair:
- Bitfinex, Link:
https://www.bitfinex.com/- Kraken, Link:
https://www.kraken.com/There is another route, however, you can buy BTC with USD (tons of options), then transact it with USDT on Poloniex or Bittrex. But fees would kill you.
I do not see Tether trading on Bitfinex, only tether deposits/withdrawals.
ON Bittrex I see a USDT/BTC market with good volume. Also USDT/ETH.
Though how to get the USDT? Does a USD deposit at Bittrex become USDT?
from bittrex.com
"We do offer wire transfers in the amount of $10,000 or greater for BTC and ETH purchases. If this is of interest to you please let us know in a support ticket that you would like to wire funds. Your account must be Enhanced Verified before we begin the wire transfer process."