I am a Canadian citizen (and resident) who did a 4-month internship in California under a TN visa (as a non-resident alien) in 2014. I am filing out form 8843 for tax purposes, and it is asking for the sponsor of my visa. How do I determine this? Is it just the company I worked for?
You should not fill out Form 8843. Form 8843 is for people who were "exempt individuals" (exempt from the Substantial Presence Test) for some part of the year. You were not an exempt individual at any point in the year.