I found a website (C-DAC) in India that performs prior art searches for free. But, you have to live in India to register and request for a search. Are there any other prior art search services for free?
patents.google.com has an 'find Prior art' button. Have you tried it? It doesn't do the best job but it provides helpful first steps.
Here are a few key resources: Patexia Patent Database
The United States Patent and Trademark Office (USPTO):
World Intellectual Property Organization (WIPO):
European Patent Office (EPO): EPO offers information about patents in Europe, including Espacenet, a searchable database of more than 70 million patent documents worldwide, from 1836 to today.