I've used the clear box from a dress shirt (you know - you wear them with a suit) - the plastic was really very thin and ripple free. I used it for the side windows of Fujimi 356 Porsche Carrera2 - they look great.
There are also many sources of thin clear plastic - have a look at packaging at the supermarket - there's LOTS of plastic containers - and most all of it is PETG - usually very clear and strong and thin.
Cosmetics departments also usually have a bunch of products that will have some sort of clear package.
Failing that - have a look in the yellow pages for plastics distributors - they should be able to point you in the right direction.