First, make sure they have some in stock. ("In stock but not offered.")
If they have none, but they have stock offered on the market you can go into that corporation's market orders and cancel the offer.
Then you can make a direct sale offer to your country, then go into your country's sale offers (or click the link that shows up in your game messages) and accept it. BAM, you just bought the forts.
The contract method takes a full production cycle to take effect. Sometimes two, depending on when you issue the contract.