I don't even know where to get the part. I am assuming Canadian Tire carries the load resistor you speak of.
I doubt that Canadian Tire would carry them. A bike shop that sells LED signals might, or you could go to an electronics shop and pick up a high wattage resistor there and work that into your system.
It's not to hard to do this kind of work on your bike, and you should be able to find all sorts of instructions specific to working with LED signals for your bike on a Ninja-250-specific forum. The most complicated piece of equipment you need is an under $10 digital volt-ohm meter from Canadian Tire or Princess Auto, and that piece of equipment will be useful for helping sort out any other electrical issues that may crop up in future.
Question - the right side turn signals still work, right? Do they blink really fast? When the left side started working briefly, did they blink really fast? At the same blink rate as the right side?
Reason I ask is that it's possible that you my not even have load resistors installed or incorporated within your integrated turn signals. A really fast blink rate can be a hint to that, or even just a solid on state.
Many flasher units require a certain amount of load to operate properly, and LEDs do not show the same load in a circuit as the incandescent bulbs that came stock with your bike.
It could also be that your LED signal on the left side has gone intermittent, again presenting a loading issue to your flasher unit.
Even if you don't have a volt-ohm meter, one easy test is to take a standard 1156 or 3156 bulb, connect one contact to the hot wire feeding your left signal, and connect the other contact to the bike's ground. If the left side suddenly starts blinking, it points to a loading issue, whether due to missing or defective load resistor, or defective LED turn signal.