These metal letters from Urban Outfitters
are fantastic. They are quite large at 6 x 10 x 1" and are available in some cool colours for only $14 each + postage. The have a built-in bracket on the reverse for hanging and the website says they come in different styles, although there is only one style shown.
Now I can find them on the US website, but not on the Uk site
?? So maybe they are only available in Uk stores and not online?
You could always have a look in antique shops and on car boot sales to try and find some secondhand signage. Used letters generally have a nice patina which adds character, the only downside is that you can't always get the letters you want in the same style. Here are a couple of mine...