http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/slps/menu-eng.html
Note
If you know that you will not receive your slip by the filing due date, attach a note to your paper return stating the payer’s name and address, the type of income involved, and what you are doing to get the slip. Use any stubs or statements you may have to calculate the income you have to report and any related deductions and credits you can claim. Attach the stubs or statements to your paper return. If you are filing electronically, keep all of your documents in case we ask to see them.