Sorry you are having such hassles, Derek. Given that you purchased Mavericks from the App store, it should be under your Purchased tab. Like Cory, under my Purchased items, it shows (in addition to some other apps) OS X Lion, OS X Mavericks, OS X Yosemite, and OS X El Capitan. Also, OS X Sierra (what I am using) does yet show there, as that won't happen until Apple releases the next version of the Mac OS.
So, starting with ignoring negative things (like the above post), besides following Cory's advice, you might want to first call Apple Canada and explain your situation. Given that you have a "receipt" for the purchase of Mavericks from Apple Canada, you should be able to get any succeeding Mac OS you want for free. If you do bring your Mac to an App store, and you want to upgrade to Yosemite, the folks there will most likely do the upgrade "in place". Given, though, that Mavericks has been on your machine for more than 2 years, that would not be a wise move. It would be better (and safer) to do a complete clean installation of Yosemite. One way that could be done is for you to bring an empty, formatted external drive with you, and have them install Yosemite on that device. But, I don't know if they will do that. The other ting they could do is provide you with a copy of the "Install Mac OS Yosemite" file, and place it somewhere on your machine. In fact, if you cannot get it from the App store, hopefully they would "give" you that file anyway, even if they do an installation on an external drive for you.
Again, you might want to first call them and discuss options. Also, no matter which Mac OS you want to upgrade to, you still have to deal with the possibility that for any third party applications you are using, some (or all) of them might/would require an upgrade also to be compatible with the Mac OS you are "moving" to. Maybe you could take an inventory of what third party apps you have, and let us know. We can then help you with such possible upgrades.
Now, it would be good if you let us know the following:
1. Are you making any backups to an external device? And if you are, what software are you using for that purpose?
2. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? If (hopefully) you have, with applicable software, you would also know the "health" of the internal drive inside your machine.