Where can I get a 100mm self adhesive vertical metric rule?

[Deadeye]

Where do you find these things? I spent ages looking on Ebay!

That also looks as though it would fit the bill Deadeye, and more affordable than the Richter ones....

well, I googled "vertical ruler" and then clicked the "shopping" tab in google. Then scrolled down.
This one is even better: scale all upwards (rather than half and half) and half the price!
https://www.ebay.co.uk/i/202474336126?chn=ps