The preferred source of letters of recommendation is the faculty who advised you in research projects and who taught your most advanced computing courses. We are most interested in assessments of your research potential and scholarly aptitude, and your rank and performance in the courses they taught you.
If you cannot obtain enough letters from your current or previous professors, then you may also include letters from employers. However, you should understand that in most cases such a letter will not be given the same weight as letters from your professors.
Regards
Sanjit Paliwal