Not all Contract Referrals in the game can be received for free.
The thing is that you may receive only basic Contract Referrals for entering the game every day. Once you negotiate all the basic Contracts, you will start receiving 7% bonus to your Fuel and Munition production rates.
You may check whether you are already receiving that bonus on the right side of the Contracts window.
Once the bonus unlocks, it will mean that you have only special Contracts left to negotiate. These Contracts unlock special Advanced units. Referrals for such Contracts can be purchased for IP's, at the Black Market, or through the Contracts window itself.
The IP's are extremely rare and valuable, so the only way to earn them is to compete in Tournaments.