There are methods to make your own at little cost, all you need is the internet and a printer!
Find what your looking for, photo/sign wise, use a program called Gimp2
http://www.gimp.org/( its like photo shop but free! ) and 're-size them' ( gimp allows you to re-size anything with no distortion of the image) and print out, its going to take a few attempts with the scale right but once done cut out and use some 'glue' (pva is good but you can use pritstick) glue them in the desired spot once dry give it a coat of lacquer or varnish, this keeps the image clear and now water proof!
Its always worth doing a few 'tests' when using glues and varnishes with printed items to make sure the inks not going to run, maybe even lacquer them lightly with mist coats off the boat and glue on after its dry.
It will be more work on your side then anything else but it will be wayyyyyy cheaper then buying some if they exist, how many 1:144 signs can you fit on a A4 sheet, and how much did that cost 20p? + the clear coat!