I've seen them in traditional hardware stores in the UK. When I was in Edinburgh our local one sold them, it was part of the Mica chain. You may have one local to you.
Any signwriting firm can cut you them out of sticky vinyl. They will come out of the machine with the correct spacing so you just need to lay them on the plate and peel off the backing. Should only cost you a few quid.
Cheers,
Iain