I think "OWI" in the Cap'n's fine post is a mistyped "OBI". Nevertheless, the good Cap'n is correct, as always: black spray paint is widely available in Switzerland, along with many other colours. To his advice to seek cans of spray paint closer to home, I'd add the suggestion that you refrain from flying to Australia to purchase it, as anti-graffiti laws there have made it quite difficult to purchase spray paint on the fly. Especially if you're under 18.
If you're really stuck for a can, just saunter down to the river near Wipkingen and ask whichever graffiti artist is on duty to sell you one.
HTH.