Sometimes there are lower fares available to TAs operating in India---they are not Indians-only or Rupee-only fares, since this is an international trip, but they are lower than those offered by international booking sites. You'll have to find a trustworthy Indian TA who can take credit cards, and communicate by e-mail.

I've used one in the past for a similar purpose (got an excellent fare CCU-BKK-CCU) but it was several years ago and I no longer have the records. There may be some recommendations on the indiamike.com forum.