I actually saw some at the Dollarama. They had 4x6 (2), in the frames section and a larger size in the craft section maybe 6x8 or so. They seemed have a clear palastic sheet on the front and a magnet sheet on the back and you could change the picture/artwork you placed inbetween.
I will be getting some of these, in the larger size, then I can change the sign as needed.
Penny