Ranthambore Safari booking through agents

We are unable to see any availability of gypsy for zones 1-6. Trying to book for march end which is still 2.5 months away. I have heard that many seats are booked by agents and you can book through them if unavailable on the offical website. Is that true? How do I find such agents. there are many search results on Google but how o know if they are genuine?