You can totally buy any cryptocurrency with USD. In fact, there are some wallets that supported the USD currency to buy bitcoin, Ethereum, and etc. Most of the trading and exchange platform also now support USD currency and now most of the cryptocurrency are now paired with USD currency. Aside from using exchange, you can also see some thread in the bitcointalk forum where you can exchange your USD to cryptocurrencies.
All of the big exchange platform allows it's user to buy or deposit Fiat to trade with crypto and cash out crypto into Fiat or directly to our bank account.
1. Find a cryptocurrency exchange. Go for the most popular, reputable ones like Binance, CoinBase, etc.