I am a visitor on a valid visa to the US from India. I have less than 1,000 US dollars. Can I purchase a prepaid debit card for this amount to enable me to make online purchases in the US?
Certainly. They can often be found at grocery and drug stores with the other gift cards. You can find a list of retailers on websites like Visa's locator. You'll probably want one with a Visa or MasterCard logo to ensure it is wildly accepted.
Note that these cards have a variety of limits and fees attached, sometimes including monthly fees. You may find that a retailer is not able to add the entire amount for you at once, as they'll only accept so much cash, and you'll want to review the fees to ensure you aren't charged unnecessarily.
Visa's website has a guide that explains some of the quirks of using these cards, such as understanding how restaurants may place a larger hold on your card because of tips. To use them for online purchases, a registration step is often required, as they don't have a name and billing address attached. The card should have a phone number or website you can visit to complete this step.
As noted in comments, these stores will also have gift cards for specific merchants, such as Amazon, which may be less trouble and lower cost if you know you'll be shopping at once place a lot.