Call Canon at 1-800-828-4040 and ask for the parts department. They will send you 2 screws free as a "one-time courtesy". I just did it on 3/3/2014 and they are sending me my screws.
IMPORTANT NOTE: Make sure you use loctite threadlocker (availble at home improvement stores and online) or your screws will fall back out with time. This will be the second time that I am replacing them. I bought the screws online the first time and never got around to dabbing threadlocker on them. They eventually fell out...