Quite often, I need to use a scale in an image. I have a few that I have designed myself, but I always like to know what is out there, so here are my "notes to self" on the subject:
I found the best results were to be had on archaeological suppliers websites, and also, which shouldn't really have come as a surprise, forensic scene of crime websites.
I particularly like these engraved plastic scales
And there are also some self-adhesive scales available too
These scales are mostly for archaeological use
Here is a blog post to some simple scales
And here is another post to a nice scale, complete with instructions for perspective correction