Like was said before you can make your own WWW with a regular white letter or white wall by grinding or sanding the side walls. Use a sanding disk on a grinder and go slow and carefull you can go through the white layer. It's easier to leave it on the car or have a way to slowly rotate it as you sand. You won't have a full white wall but they won't cost much.

Here's some videos.

http://www.youtube.com/watch?v=omEAPGnTXws&NR=1

http://www.youtube.com/watch?v=pHWTa2tAYIc

Ones I did.