you might want to check out
under "products" they have AccuTouch touchscreen components. im not sure what size you would need, but have a look around. there a quite a few touchscreen companies that sell components, but since im at work, im brain-dead and cant think of any other ones.
just a little trivia side note, the impee3 car website is the site that really got me interested in this whole car mp3 player culture. just a little ego building praise for you...assuming that is your page in your sig!