I have a Master’s degree, and want to apply for PhD. I want to find a position during the gap year. It is also possible to do 2 years. But I am not in USA. So, How do I find a research assistant positions? Is unpaid job possible to get visa?
Research Assistant positions are almost exclusively filled by graduate (and occasionally, undergraduate) students. They are not normally available for non-students. You'll have to wait until you're enrolled - and an RA position may be part of your financial aid package.
Research Scientists and Research Staff positions are available to non-students. However, because there are so many people with work status who can fill them, employers will rarely agree to sponsor someone for a visa for them (unless there is a mitigating circumstance, like a spousal hire). You can find staff openings through the human resource (HR) portals of most major universities.
Under US law, you generally cannot take an unpaid job to circumvent visa restrictions -- this part of your question is best asked and answered in expatriates.stackexchange.com , as I have seen you have done.