It is true that many programs will simply spell it out for you and require letters from specific people (like a direct supervisor, a peer, etc.) But, if your program does not do that for you pick some people whom you feel will give the most honest and favorable representation of you and your ability to succeed. I asked a CRNA who really didn't know me that well on a personal or professional level, but who I had shadowed several times in the OR. I like what he did...one evening he called me and just asked me several questions to fill in the blanks on what he didn't know about me. He asked what I have done to prepare adequately for CRNA school, what types of patients I routinely care for, and my educational background. I also asked several doctors I worked with as well as some professors from my undergraduate program. I made it a point to give my cover letter and resume to everyone I asked to write me a letter. This is helpful to them especially when they may not know you as well as you would like. One final point is to ask people WAY in advance because some may not be as timely as you would like and you don't want to miss deadlines because they were slow about writing a letter for you. Hope this helps, and good luck!