You can book them on eazyPNR, where sellers list FD & Series Fares (Fixed Departures and Series Fares) on the Kuala Lumpur to Chennai route and travel agents book them for their customers. Each seller sets its own agent fare and payment terms, and both are shown before you confirm.
The flow is simple: register, search KUL to MAA by date, compare sellers and seats left, enter the passenger names and confirm. Booking is instant where the seller allows it, otherwise it goes as a request and the answer arrives in your account with the PNR. Release dates and change rules depend on the seller's airline contract, so read the fare rules on each date.
Current dates are on the Kuala Lumpur to Chennai fixed departures page. To see fares, register.