Working nights in a grocery store

That's what I'm doing now, and I'm wondering if there would be any stores in Japan that would hire foreigners for the graveyard shift. Stocking shelves is not super fun, but it has advantages, and it's easy.

If you have a visa that allows you to work in any job in Japan (i.e. spouse, working holiday, student) then you could get these types of jobs.

However, a work visa would not be an option, as this job would not only not satisfy the requirements of the visa, but you even if they could hire you no one would want to go through the trouble to sponsor a visa when they could just hire someone locally. Basically, you would have to be more qualified and desirable than a local Japanese to even be considered, and this simply isn't the case for non-professional jobs.

I guess being Canadian is a good advantage then!

Definitely, as a working holiday visa is eligible for these types of jobs. However you may still have difficulty securing a position. Why not consider other jobs, like teaching for example, that probably pay better and would be easier to get.

As a side note, I used to work at a grocery store in the US, and the Japanese grocery stores, at least in my area, don't seem to have a graveyard shift, preferring to stock during the day.

Well I'm guessing that being 18 with only a high school diploma won't get me teaching anyone. I think I'll have to go to university after all. But do I have to study a particular field, because I don't think a bachelor's in Psychology would help me teach English...

Actually any degree would help you. For example, it would qualify you for the JET program.
http://www.jetprogramme.org/e/aspiring/eligibility.html

Great! I guess I won't be seeing Japan until I'm like 24 though :(

why would you even want to do that? what a waste of time. stocking grocery store shelves is at the bottom of just about every social ladder no matter what your age, furthermore if you speak english why would you even want to consider that when you could be making double or triple the salary for even less work!?

ridiculous! if you want to stock shelves why bother leaving canada.

Well the secret behind it, is that when you're working at night stocking shelves, chances are you can get away with wearing earphones! I listen to my Ipod, making the night a breeze!

For help in finding a job in Japan see
http://www.tfemploy.go.jp/index_en.html
You can also try Hello Work
http://www.tokyo-icc.jp/guide_eng/gov/04.html