|I live in the Philippines and use a local company called coins.ph.|
In the US you cannot buy real Bitcoin via a normal brokerage account.
Popular companies which do USD to Bitcoin exchange include CoinBase and something called Kraken (I think). Setting up an account is a bit more cumbersome than setting up a brokerage account, but it's not too hard.
If you're interested I would suggest opening a Coinbase account and just putting a small amount of money in to learn about things. Crypto today reminds me a lot of the Internet stocks in the late 1990's, only earlier and wilder. There are lots of alt coins that are trying to do stuff, many of them scams, other real investable entities, and with about zero regulation and oversight by the "authorities".
There is a US stock - GBTC - which is a trust that just holds Bitcoin. So it tends to trade in tandem with the price of Bitcoin, but it tends also to trade at a large premium to the actual Bitcoin held. I own some of it. Super volatile, but it has gone from $103 up to $1000 and now down to $620, all in less than a year!