Bookings for all trains open 90 days prior to the date of Start of train from Originating station.For the Mumbai Delhi and the August Kranthi Rajadhani it will begin 90 days before your date of journey.If you are planning to use the TVC-NZM Rajdhani it will have to 91 days before date of journey.
You have a lot of tour operators in Delhi, but in case you need any specfic assistance drop me a line,I could give you suggestions since i have traveled across most of India on my trips.