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].
[1] https://coinmarketcap.com/currencies/bitcoin/#markets
[2] https://coinatmradar.com/