I would like to know if I could get somewhere some kind of stickers, which I could use as an alternative to spray color with defined emissivity to measure copper and aluminum surface temperatures with pyrometer.
I think almost any paint with a matte (vs glossy) finish has high emissivity. At your hardware store you should be able to find Krylon flat black spray paint that has an emissivity of around 0.95. You can of course check the emissivity for yourself without too much trouble.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Meanwhile I have found the supplier in Czech republic. The following link is in Czech but i attach it just to have and idea about special color and stickers with the defined emissivity and able to withstand high temperatures: