I graduated last year and am looking to go to grad school for next year. I'm pretty sure I can get a strong letter from my undergrad thesis supervisor, he has told me he would write me one. But other than that, I really had no relationships with profs. I did pretty mediocre in my first two years and well in my last two years but I didn't make much of an effort to get to know my profs. Overall my grades are good-ish. I definitely meet the cutoffs for the grad programs I am looking into but I didn't really do stellar in any particular class, much less have the prof remember me. Looking back I regret the way I went through my undergrad but I know there is nothing I can do about it now. I just didn't know I wanted to go to grad school until it was too late. The only other person I can think of for a letter is the PhD student I worked with/who's research my thesis was based on, who is now a sessional prof. Would this be just as god as a professor letter? I just don't know who else to ask.

It depends on the grad school. If there isn't anyone else I'm sure the PhD student will be fine, I know some grad schools consider recommendation letters from TAs as "academic recommendations," although letters from profs are obviously preferred. If TAs are okay I don't see why a PhD student wouldn't be.

That being said, it wouldn't hurt to ask a prof. My roommate got a recommendation letter from a prof she barely knew, but she did pretty well in his class so he wrote her one anyways. When she asked him he told her that he would first have to ask her TA how she did in tutorial, and wrote her letter based on the TAs recommendation / comments.

You don't always need to have a relationship with the prof, as long as you've taken a couple classes with them or did really well in one of their classes you should be fine. Just remind them what classes you took with them, when you took them, and the grade you got. Worst case scenario they email back and say they can't give you a reference, it won't even be awkward since you already graduated so you won't even have to bump into them on campus afterwards.