Unfortunately direct USD to IOTA isn't available yet, you will need to buy another crypto-currency first, transfer it to an exchange that trades IOTA and trade it there. The best/cheapest way to buy IOTA differs from moment to moment and can be found on https://iotaprices.com/ - This includes instructions on how to do so. Buying Ethereum is currently ...
How to Purchase IOTA in the US
Overview on how to purchase IOTA in November of 2017 in the US. For the sake of simplicity I am going to use Ethereum (ETH) as an example. Bitcoin (BTC) could also be used here (w/ higher fees).
Create a Coinbase Account (recommend 2-factor authentication)
Create a GDAX Account (owned by Coinbase, same market)
Link a new bank ...
1) Buy BTC or LTC from CoinBase
2) Transfer BTC or LTC to CoinSpot
3) Buy IOTA with BTC through CoinSpot
4) Send IOTA from CoinSpot to your IOTA wallet
(CoinSpot is an Australian exchange you might be more comfortable using as a US citizen than an Asian based exchange)
I think you should buy some bitcoin or ether on any exchange, and then You can buy IOTA tokens on Bitfinex.com or www.binance.com. Is what i did actually I've bought some ether on coinhouse.com and then use it to buy IOTA on bitfinex
At the moment there are no exchanges that exchange USD for IOTA. Your best (also fastest and cheapest) method would be to exchange USD for LTC on Coinbase, then send the LTC to an exchange that trades LTC, BTC, and IOTA such as Bittrex or Binance. Once your LTC is moved from Coinbase to the other exchange, sell the LTC for BTC, then use the BTC to purchase ...