Though you can book online directly from airlines company but sometime offline booking or booking through a local travel agent / travel agency could fetch you good discount as they run some promotional offer. I found flightoworld.in which is based in lakhmi nager who is currently offering the cheapest airfare for flight to Bangkok with upto 20% discount. They also promise to provide you with a immediate booking and quick refund. Try flightoworld.in
Don't forget to book a taxi from Delhi airport.