Other than coming across a used piece of gear that you might not have found locally, I don't think there's any reason to shop for a bass while in Canada... You can get everything & more from the big online retailers Stateside (with free shipping)...
If you've got your eye on a custom order F-Bass (stopping by their Hamilton shop to place your order or pick-up a finished instrument - is tres cool!)... They have some pre-owned instruments, but not in the $500.00 range... & definitely contact Fbass before stopping by...
If you have mobile internet access, google map "musical instruments" on your travels to find local stores...
If you cross the border @ Detroit / Windsor (you'll be travelling south to Canada - ***?) stop by The Economy Exchange Pawn Shop (apparently the owner loves guitars)...
Drop by any Long & Mcquade that you pass - there are 2 in London... & lots along the 401 (but you'll have to stray off-path & could waste travel time) - there are shops in Guelph, Kitchener & Cambridge - walk in with a "good morning" (or afternoon - whatever the case) & you'll likely get some friendly Canadian hospitality... While in the Toronto area, you'll likely get a slightly different attitude

& in Montreal, you'll be greeted in French first (it's the law)...
While in the big smoke (Toronto) - I'd visit any area L&M, Steve's Music (on Queen St West), Capsule Music (also on Queen), maybe stop into MOOG audio (since it's on Queen too)... Club Bass is a great place (on the Queensway), but not for the $500.00 budget & not too much in the way of used gear... Also, Paul's Boutique (in Kensington Market - near Chinatown), the 12th fret (on Danforth, not far from Greektown) & Just west of Toronto (on Lakeshore Rd East - in Mississauga) L.A. Music & the Guitar Shop are just across the street from each other...
Just north of Toronto, there's Cosmo Music (maybe the largest one location music shop in Canada - but I could be wrong)...
If you get to Ottawa (you likely won't, it's a few hours out of the way, between Toronto & Montreal) - Check out Spaceman Music, they also have L&M & Steve's...
In Montreal, Steve's & Jack's Musique are quite close together on Rue Saint Antoine Ouest (near Old Montreal)... Italmélodie & maybe Archambault (not so much gear, but lots of French Canadian CD's)...
Some of the shops mentioned will be a waste of time - but ya never know...
Enjoy your stay in Canada & make sure to try some local delicacies - Great German food /beer in the Kitchener area - Lots to eat in Toronto (look for some authentic Neapolitan pizza, Italian, Greek or Chinese - depending on the neighbourhood)... Definitely try the Poutine and smoked meat in Montreal...