<< I know I've seen large
transparencies at trade shows, but they usually have a white opaque
backing... which I don't want. I want a clear normal transparency... large.
>>
Any pro lab or custom color lab should be able to do the enlargement for you.
The one white white backing is called duratrans, whereas the one with clear
back is called duraclear.
However, these transparencies are meant to be the final products, that is,
they cannot be used for further enlargment.