Is such temporary health Insurance for visitors available?( 3 - 6 months )
Any recommendations of such Health Insurance Providers?
What could the monthly premium be like?

Is it worth buying it?

Please advice and share experience.

You can buy travellers insurance in Nepal before coming to US. This is cheaper but only covers emergency services. You can always buy health insurance for parents but it will be very expensive for older people. The older you get the more expensive it becomes. You can always call the insurance agents/company and ask them about the cost.

For what it's worth, your car insurance probably has an option which covers all occupants in the car should, God forbid, an accident occur.

Ashish - Check this site out : http://www.buyamericaninsurance.com/?affiliateid=x

Health insurance is especially recomended if parents have health conditions that may require unplanned visits to hospitals.

Ashish!
There are some short term Health insurance providers. Some states may not have it. So it depends on where you are. I recently bought a short term insurance for this summer through blue cross blue shield. The premium is less then the regular health insurance but they have 5 years pre-exisitng conditions. If your parents are 65+, then it does not work. You might want to see their wabe page too.

Good-Luck