They're all full alphabets, as far as I can see.
The ones from Parsimonioius are quite nice. If you recolor them black it might work. The only thing is that they're a little angled.
The suza letters come in an entire alphabet, so recoloring them black would probably work. They're not for walls, but you can place them with OMSPs and cheats.
Here's another set of letters (go to comments):
http://www.modthesims.info/download.php?t=213739&c=1