Letters of Recommendation should are highly significant documents in the application process, and you should invest in them as they can be quite influential in your admission.
Individuals who have a good deal of knowledge regarding your abilities, personality, achievements and potentials, and with whom you have had a high level of interaction during your previous academic or professional career, can be suitable for writing and signing your letters of recommendation. These individuals include previous professors and instructors, thesis/dissertation supervisors, dean of school/faculty etc.