The first time I had to replace a damaged tent in San Diego was quite a crisis. The downtown thrift stores sometimes carry tents but it is not a sure thing. I have never bought a tent from any of them. I could not find any other downtown stores that carry tents. You basically need to leave the downtown area to buy a tent.
When I went to the Kmart here, I was told they do not carry tents. I am dumbfounded. I have bought tents at other Kmarts.
There are several Walmarts in San Diego county. They are all a very long walk from the Gaslamp District but they can usually be reached by bus in a reasonable amount of time. The first few times, I walked it: One day to walk out, sleep nearby, and one day to walk back. These days I spring for a bus pass for the day.
You can also sometimes find small tents at Target.
If you have no money for a tent, I have been told that the Lutheran Church on First and Ash has a sign up on Mondays for tents and blankets.
I have a small income and a serious medical condition. It is important that my things are very clean. As much as possible, I try to buy new but cheap stuff. Getting well is the only hope I have of getting off the street so my health is my highest priority. I have shopped the thrift stores and gotten free stuff from various services but I am currently trying to avoid that so I can continue getting healthier.
I mention that because a lot of people on the street are there in part because of medical issues. Getting healthier is causing my expenses to go down and my productivity to go up. So my plan in that regard is working, even though it is slow going.