Here's a list [1] of 400 exchanges you can buy Bitcoin from, 34 of trade BTC/USD. This doesn't include some sites that charge a premium and allow you to buy with debit cards. There are also Bitcoin ATMs, over 1000 of them in the United States [2].
That's not a useful response. ATM's will likely have over 1-2% in fees, and just trading USD isn't enough - you need a way to get your money in and out.