Please recommend reliable travel agencies for USA travel visa. We are planning to invite my uncle & aunt (mom sister) for holidays, thanks
Don't use any agency: just apply directly through the US consulate in Chennai.
Click here for the application form. The visa they need is the B1/B2 visa, which, for Indians, is usually valid for 10 years (though less may be issued) and lets you stay for 6 months per visit.