It depends on your choice, if you're buying bitcoin, you can make use of a centralized exchange or a decentralized exchange p2p, alternatively, you can as well get any other merchant trusted to sell or buy form you online or in physical, but they must be trusted or better to use an escrow service provider, which is the major function or role an exchange play when they are selling or buying for you, except you wanted to receive it as a gift or mine it yourself.