That's actually a really good question. Do you have a Disney Store near you? That is the only place that I think still sells them other than the box office at the park.
I know my college ticket office sells them. I wonder if that's a possibility for you to check. The upside of buying them there is they're about $5 cheaper than at the entrance. I don't know if only students can buy them, that might vary by school.
single day single park tickets are only available to the general public(not sure about those student tickets cuz my school doesnt have them yet) via the ticket booth or by calling 714 781 4400