Running a rental takes a small amount of paperwork, and the supplies that go with it are a rental expense. On Form T776, they go on line 8810, Office expenses.
The line is narrower than the name suggests. It's for consumables — things you use up — not for the furniture and equipment you use them on.
What belongs here
Small items such as pens, pencils, paper clips, stationery, and stamps.
Think of the stamp on the rent receipt you mailed, or the paper you printed the lease on.
What does not belong here
Capital purchases such as calculators, filing cabinets, chairs, and desks. These are capital items: they last, so instead of coming off this year's income all at once, their cost is spread over the years you own them through capital cost allowance (CCA). That's handled on a different part of the form.
The dividing question is whether the thing gets used up or gets kept. A box of envelopes is used up. A filing cabinet is kept.
Dossier flags a purchase that sounds durable so the question is in front of you when you file. Dossier doesn't answer it.
What to keep
Keep detailed records of the rental income you earn and the expenses you incur, supported by invoices, receipts, contracts, and other supporting documents. For this line the receipt itself is usually enough, as long as it shows what was bought — an itemized office-supply receipt answers the consumable-or-capital question on its face, where a card slip for the same store does not.
Don't send them in with your return — keep them in case the Canada Revenue Agency (CRA) asks to see them. The CRA may not allow an expense you can't support with a document. Generally, keep your records for six years from the end of the tax year they relate to.
Official CRA sources
Source: Canada Revenue Agency (canada.ca). Not tax advice.