miniature LED 7 Segment

The closest you will likely find over the counter is to gut some early (70s-80s) era handheld calculator for their vintage LED displays... They won't be 1mm x 2mm but they will be small... Many of them used very small 7 segment LED mounted behind magnifier bubbles, if you can remove the bubble the display is even smaller but it also might be what accounts to an early CoB style mounting on the PC board that could be a real pain to remove and work with... If you dig around you can sometimes find NOS displays in the traditional DIP sized package, complete with the magnifier bubble...

A 1mm x 2mm seven segment would in all honestly be a single blur to most people except at very close distances unless a magnifier was used...