Hey... as gamecreature mentioned (thanks gc!), I think I can help you out, too.
If this is considered spammy, let me know and I'll delete ASAP.
I do a lot of custom cut vinyl for forum members and if you have an idea of how you want it laid out, I can create completely opaque ready to apply vinyl decals. You don't really need transparent vinyl if you just cut the insert out of any normal opaque vinyl. All the empty spots would be the parts where the light shines through.
Here's the thread again:
http://forum.arcadecontrols.com/index.php?topic=51813.0 Even though those are on discs, I do offer everything in any form I'm capable of. So if you can manage some type of blank housing, I can cut you some stickers to throw in there. International shipping is no problem either.
The problem I've had with transparency paper is it's never really opaque enough. Even with all the various laser printers I've tried it's still blotchy and translucent when backlit, especially with a superbright LED. But that is a great budget/diy option if you don't mind it.
Hope that helps!
mike