Independent references
This is not the time to be shy; if you feel your work is good enough, you shouldn't be worried about approaching scientists you don't know for references. Here are some pointers:
1. Ask your current boss and former boss(es) if they can possibly ask their colleagues/friends to write letters for you. Oftentimes, their colleagues have already been asked to write such letters; and sometimes your own boss has written such letters for his/her colleagues, so it's payback time.
2. Look at your citations, and see if there're labs/groups that frequently cite your work. Write to the corresponding author, and ask him/her if it's possible to write a letter for you about the vital importance of your work in their own projects.
3. Do not ask your own junior scientist friends for letters, especially those from your own home country. The INS will sense that these letters may have been written only because of a personal relationship; moreover, they're probably not senior enough to be believable.
4. Only include VERY STRONG letters: i.e., those that say "his work is among the best I've seen in my career", and not "his work is of high quality." The common mistake is to dilute the application with not-so strong letters, hiding the influence of the really good ones.
5. Don't forget to include your recommender's CV with their letters.
GOOD LUCK!!!