An immigration agent is normally for a country that is actively seeking immigrants (usually immigrants with specific skills or professions). The US isn't looking for immigrants, so I would be extremely wary of anyone claiming to be an "agent" for US immigration.
For securing a US visa, you normally would use an immigration attorney if you have some specific problem or issue related to a particular category of visa. They can't really help you if you have nothing to base a visa application on.
If you succeed in finding a potential employer, they will sponsor your application and they will be the ones to consult their legal advisors for the best way to proceed.
As far as finding an employer in the US, good luck. Your best chance is if you have some particular talent, experience or training that is in short supply in the US. There is no list of these niches. The employer will have to justify hiring a foreigner needing a visa over hiring someone local for the same job, and the process of sponsoring a visa is not free for the employer.
Job hunting is a marketing project in the US. It's up to you to "sell" yourself to a potential employer - what you can do for the company and what special competences you bring to the deal. Make sure that your field of expertise and your academic credentials are acknowledged in the US. Many professions are licensed in the US at the state level and without state qualifications you won't be considered for the job.
Don't mean to be a killjoy but the job market in the US is pretty brutal and being a foreigner needing a visa puts you at a distinct disadvantage in the job market.
Cheers,
Bev