The back glass on the iPhone is very easy to change, you just need to take out the two screws at the bottom of the phone and slide it off and insert the new one. I couldn't find a blue one, but here is a red one and a white one:
Red: http://www.ebay.com/itm/NEW-OEM-iPhone-4S-CDMA-Color-Back-Glass-Cover-Red-/300973855390?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item461370929e
White: http://www.ebay.com/itm/NEW-WHITE-iPhone-4S-Back-Glass-Rear-Battery-Cover-GSM-CDMA-ALL-Versions-/281234816832?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item417ae6bf40
If you buy the back glass you will need this screwdriver to take out the screws: http://www.ebay.com/itm/Repair-Tool-Kit-open-5-Point-Star-Pentalobe-Torx-Screwdriver-iPhone-4-4G-5-8pc-/251338139796?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item3a84eba894
All of the above ship to Canada and eBay is a trusted website. I have been purchasing things from eBay for as long as I can remember I have stuff coming in the post all the time. If anything goes wrong with an order on eBay, you can get a refund no matter what for the first week after receiving the item even if the seller doesn't state it, that is how eBay's buyer protection plan works, so don't worry about it not being trusted, eBay is a great site.
As for the front, do not get a new screen, it is a hard job to replace and it isn't as cheap as the back cover as you will need to buy a new LCD. I'm sure you can find some cheap sticker borders on eBay if you take a look around. You can also check out this website: http://www.domeskin.com/iPhone-4-Skins if you don't want to replace the back glass. I have purchased from here before for clients, it's a great site.
Good luck! I hope you are happy with your final result!