It used to be a lot easier in the past but now the United States is flooded with way too many STEM professionals, all desperate to achieve permanent resident status. Not too long just a PhD was enough to impress the USCIS but not any more and your inability to find a sponsor is definitely a red flag. Also remember that many applicants with sponsors use the NIW route as well because they are qualified and their employers just want to make sure that the permanent resident status is granted.
Here are the requirements of which you need to meet at least three of them in addition to proving that you have exceptional ability (a degree of expertise significantly above that ordinarily encountered in the sciences, arts, or business):
- Official academic record showing that you have a degree, diploma, certificate, or similar award from a college, university, school, or other institution of learning relating to your area of exceptional ability
- Letters documenting at least 10 years of full-time experience in your occupation
- A license to practice your profession or certification for your profession or occupation
- Evidence that you have commanded a salary or other remuneration for services that demonstrates your exceptional ability
- Membership in a professional association(s)
- Recognition for your achievements and significant contributions to your industry or field by your peers, government entities, professional or business organizations
- Other comparable evidence of eligibility is also acceptable.
As you will notice some of these requirements are somewhat subjective. Therefore, I strongly suggest that you start interviewing with law firms that specialize in National Interest Waiver petitions (a good place to start is your peers) because a regular attorney may not have the skills needed to build a case that somehow you are so special that your presence in the United States is of national interest and you should be granted a waiver because Americans or other permanent residents are not available (remember that a lot of your peers in biochemistry are in the green card pipeline but have sponsorships from their employers). In other words, the bar is quite high for you in proving that you are superior to all of them. You have just a few hundred dollars to lose though if you apply and are rejected but if you succeed, you are all set.