Unfortunately Mary, the short answer is that you cannot.
This is a very basic model printer, and as a 2012 model it is already well past its service life. All support from canon is long discontinued, and for these basic models even when new they don't make those types of parts available for them. Even if the part were available, it wouldn't be worth investing in as the part alone would be nearly as much as a new printer.
If a broken/missing cover is preventing you from using the unit effectively, you'd be better off to just buy a whole new system at this point. Because even if you did have access to the part, other components of the printer like the printhead and absorber system will fail in the near future, ultimately requiring replacement of the printer anyway.
If looking for a replacement, I’d recommend any of the Epson workforce series printers. They tend to be very reliable, are easy to set up and use, have good print quality, and reasonably priced ink. Example: http://amzn.to/29wF4OP or http://amzn.to/1NYUF1P
Note that these, like all consumer grade inkjet printers, are designed to be low cost and are essentially disposable after a few years, subsidized by the high cost of ink cartridges. If you wish to have a better quality and longer lasting printer, that comes with years worth of ink up front, consider the Epson ecotank models instead: http://amzn.to/1PpOj0C
Alternately, for a more business/commercial grade product that will be rated for a 5 year life, consider a laser printer. Lasers cost more up front also, but a toner cartridge can last for thousands of pages. A black only all in one laser is around $125: http://amzn.to/1JSHOlQ
while a color model is about twice that: http://amzn.to/1n4I7Q5
My apologies for the news. I do wish there were a simple fix for this, but unfortunately the standard to which canon designs and builds these printers is one that does not lend itself to economical repair options.
If you have any other questions, just let me know. Otherwise, please take a moment to rate my service using the stars at the top of the page; hopefully I have earned 5 Stars, even if the news was not what you were hoping for. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh and ask for Nathan.
Thank you, ***** ***** a nice day,