Overview: Obtaining a Green Card without an Employer Sponsor
For the bulk of foreign nationals, there are 2 main classifications of options when seeking a permit: family-based and employment-based. For individuals who do not have an immediate family member who is a U.S. person or Legal Permanent Resident, family-based options are either impossible or featured a lots of years-long wait.
Employment-based options can be more broken down into two categories: employer-sponsored and non-employer-sponsored (likewise referred to as self-petitions). Employer-sponsored options are the more common of the 2; they consist of the Labor Certification process, which is appropriate for all tasks, and Outstanding Researcher/Professor (EB-1B), which is only applicable for tenure track or permanent professors or research study positions. The only 2 employment-based immigrant visa categories where a foreign nationwide might self-petition are National Interest Waiver (EB-2) and Extraordinary Ability (EB-1).
Many employers have limits as to who and when they will sponsor for irreversible residence. They might just supply sponsorship for specific positions, or employees who will remain in a position for more than a defined length of time. Alternatively, an employer might have a "waiting period" in which employees are not eligible for sponsorship up until they have been with the company or institution for a particular length of time on a short-term visa.
Positions that are short-lived by nature (such as postdocs, medical residents/fellows, or checking out professors) or part-time will not be suitable for employer-sponsored classifications.
If you are investigating long-term residence categories that do not need employer sponsorship (i.e. 'self-petitions'), note that your opportunities and certifications for these classifications will enhance as your profession progresses.