It is used to bundle wires and exists in diameters from about 2 mm up to ? (I only have it up to 50 mm).
The diameter is measured in "rest" (unstretched and uncompressed).
You can get it in well assorted electronics shops (on- and offline) and I have gotten it from PC-modder shops as well (even in Fluorescent and phosphorescent (i.e. glow-in-the-dark).
For air-muscles, the color probably don't matter.
For the bellow, inner tires can be had in several diameters (racing bicycles to tractor size).