Most local USPS Post Offices sell pre-stamped postcards (stamped cards) and occasional sets of decorative postcards. You can also find a wider variety, including themed or commemorative sets, on the USPS online store .
If you buy a plain postcard, you will need to purchase separate postage. can you buy postcards at the post office
Most post offices in Canada offer postcards, often with the necessary stamps available at the same counter. Most local USPS Post Offices sell pre-stamped postcards