I second PET-G but be aware that it'll go brown over time, but then again, so will acetate.
I buy 8'x4' sheets of 0.5mm PET-G plastic from my local plastic guys, and use it for canopies in a vac former. A sheet that big will usually last me about 5 years, and works out to about $2 a canopy. At that price you can pull a few and choose the best, or just mess up more than once!