Do you have stairs in your house? Stand with your heels hanging over the edge, let them drop all the way down and hold for two seconds then bring them back up and hold for another two seconds. Keep doing it until you can no longer do them! I do them religiously 3x a day and swear by this workout for improving my calf muscles which will help you at the canter.
If you want a real workout, do the same exercise but with one foot at a time - so that your entire body weight is on that one foot as you stretch it down and bring it back up. My left leg was also weaker at one point and I worked that leg a lot more than the right leg.
There's a video showing how she does it, but I like to wait 2 seconds before bringing them back up. Of course, more riding will also help you! Lots of two point, especially two point without stirrups is good for strengthening.