Factories that could make off-patent bricks? Have you tried asking about this at a Lego store? They almost always have basic 2x4 bricks in the PaB wall, and I'm pretty sure they are able to order them by the case for you.
Why can't I reproduce this SD card box in LDD? You are correct that .12mm is the same as .3 LDU, and this does seem like a strange offset. My guess is that this decision was made in order to create the absolute minimum difference between these parts. I'm guessing that most children who build with these parts have never noticed this minor inconsistency.