How to write a cover letter if you have no contact info???

How do you address a cover letter if you're applying to an online job posting and don't know the name of the recruiter? The listing of recruiters that someone posted was VERY helpful but only covers a tiny percentage of hospitals/recruiters. Also, one of my professors will not give me a letter of recommendation unless I provide her with contact information for the person reading it.
It's just fine to use "To Whom It May Concern" if you do not have a specific name.Rather than a letter of reference just state "references upon request" at the end of your resume. If they want to get more information, they will ask you for the names of your references & contact them directly.