Typically professors and employers (or other supervisors) write LORs. However, if your TA is very familiar with your work and you can trust them to write a persuasive LOR, the TA might be a good option. That being said, you might now want a LOR from just another college student without some other professional background. You want to be sure that the LOR comes from someone who can write effectively and has the background or other proficiency necessary to be respected by the committee who reads the LOR. Good luck!