Whatever kind of paper product you're looking for, you'll find it at Paper Plus. A little establishment right in an industrial section of El Cajon, it looks like nothing from the outside. Upon entering it's nothing classy, just shelves of paper products at great prices. You buy in bulk but save so much.
Any size of ziploc bags that you're looking for, they have here. Same with poster mailing tubes and boxes. You can also buy huge rolls of bubble wrap here at great prices compared to other retail stores.