Yeah but not all of them are selling order flow. E.g. Vanguard and Fidelity don't sell order flow and haven't blocked purchases of these stocks. They make their money the same way banks do.
Can one buy stocks through Vanguard?
Yes, Vanguard supports individual stocks with no trading fees.
And chances are if you're trading large blocks you have a lot of money, making you a premium customer, meaning you get free trades anyway!