If you would like to place an order for huge amounts of BTC at once, you will have two options. Choosing a centralized auction-based platform such as the exchange. The advantage of this method is availability – exchanges are numerous, and bigger ones will likely have enough cryptocurrency to maintain the liquidity. They are also safe, trustworthy, and compliant with regulations. The drawback is that exchanges commonly have restrictive caps on daily traded amounts, the orders placed are visible for everyone, which is considered a huge threat to the market price.
Another option for you is an OTC desk or broker. Perfect for clients with large amounts of funds, both crypto, and fiat, and, as a result, have much higher limits on trades. OTC desks keep the privacy of the orders, do not influence the market price, the process is conveniently handled almost entirely by the broker for your convenience.