In my experience grocery stores almost always have positions available for after school work, I don't know how old you are but pubs, clubs and hotels are always worth checking as well I know in Aus they're the sorts of places that get students so they're are used to casual staff and high staff turnovers.
I don't know about working from home, but you could always just steal shit and sell it on ebay...
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And I don't know if you've already thought about it/tried it, but maybe talk to your parents, tell them your saving for a computer and maybe see if you can help them more around the resurant for a little extra money. Like when I was younger and me or my brothers wanted something dad would say "ok I'll give you x amount of money if you guys help me do this -, or fix this - etc."