Scotch Indoor Mounting Tape, Holds up to 6 pounds, 1×1 inch, 48 squares – Link

Amazon has this marked down to $2.97.

• Double-coated 1-inch foam squares
• Adhere and conform to a variety of surfaces
• Ideal for mounting pictures and decorations on most types of walls
• Permanently bonds to most surfaces
• Squares hold up to 2 pounds