If you've no luck finding them tucked away at big box stores, or hidden away in the back of an EBgames (ie; ask, they won't have them on display.. make them look it up in the computer), then you could check out A&C Games when you're in Toronto. According to their late September count, they had new and used. And the used even had boxes for most. A&C can be a little pricey though. E-Mail and ask for a price. (link below)
Other than that, call Game Centre (416) 925-4951 (located at Yonge and Charles) and ask them to call you when they get a Micro in. They have them in pretty frequently, but you'll have to check to make sure condition is good. I spent six, or seven months waiting for them to have a mint used SP show up, even though they have 2-3 in weekly. Of course.. I'm picky.
Amazon, Craigslist, Kijiji and Ebay are full of them, but if you want to save a bit and get one with less hassle, then try garage sales. I would suggest settling for an SP or Orignal GBA though, because Micro's are harder to find and more expensive.