Adhesive backed magnetic tape 25.4mm wide sold by the metre.
The market leader at 1.5mm thick.
Widely used in:-
To name but a few
Easily cut to required size with Scissors or craft knife.
If you wish to stick 2 non metal surfaces together you will need magnet on both surfaces, for this type of application please order both side A and Side B magnet as required.
If using this magnet to attach something to a metal surface then either side works(For example a fridge magnet).