Coinbase is one of the few well known places to buy that keeps an actual US Dollars (USD) Wallet. They charge you out the ass to buy directly through them, but sometimes you gotta do what you gotta do. If you use your Bank Card instead of your Bank Account to buy, it is instant. They do have a much lower limit of how much you can deposit with each card per week, but it is instantaneous. Most exchanges don't keep a USD Wallet...this keeps a separation between the USD and the exchange, and keeps from possibly infringing on US laws. Instead, most markets have USDTether (USDT) Wallets. This currency is supposed to be backed by real US dollars, and keeps within a couple cents of $1.00 USD at all times. I believe Kraken lets you buy and keep USD, but I don't use Kraken, so I'm not 100% sure on that. Here is an article explaining USDT: https://www.cryptocompare.com/coins/guides/what-is-usdt-and-how-to-use-it/