You can do it yourself

NIW is no need to be sponsered by boss, who, if willing, only offer you a recommendation letter. That is all. If not willing, you can get letter from others. Be careful! Some bosses use GC to control their employee. I know a chinese professor, who told his chinese postdoc to work in his lab with pretty low pay and promised to help the postdoc petition NIW GC. Ridiculous!