Yes, you should try to get letters of support from relevant business contacts who have credibility in the field you are working in. For instance, you may be applying for a grant in clean energy based on research from your Ph.D. work at a university. You may have collaborated with a faculty member in another university, or a corporate sponsor who funded this research.  A letter of support from either the faculty member or a leader within the corporate sponsor would be very helpful. The letter does not need to be overly specific; all it needs to do is acknowledge how the other party worked with you and how they know you are the best person / best company possible to lead this research effort.