When I need tiny digits for things such as serial numbers, I cut out the little placement numbers on decal sheets that identify where markings should be placed on the model.
They come in all sorts of fonts and sizes, and many were treated with the rest of the sheet and have decal film on them.
The two brands I turn to most often are Academy and Airfix.
Tip submitted by:
Alex Sweatman (age 12)
Rankin Park, New South Wales, Australia