If you want the heavy duty type safety signs, then I'd say look in the local Yellow Pages for Signwriters, or some Stationers can have them made up &/or order them for you.
If you only want the lighter ones, you could buy a cheap laminator & make up your own safety signs. That way you can type up whatever you like, print them out & laminate them - A4 or otherwise.
I used to make up signs up this way at work & still use a laminator at home - comes in very handy, 'specially to encapsulate the Grandchildren's 'special drawings, etc!