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Sam is a trusted home cook.
Try a good Graphics Art store..not a craft store. If you looked in a craft store, you should probably not look in the cooking section, but the framing, paint, and paper section for graphic work.
Sarah is a trusted source on General Cooking.
If you're not in a rush I'd go online to something like this:
(Never shopped on this site before, but gives you an idea of the price range - acetate isn't cheap.)
Depending on the size you're seeking you might find it in a place that sells old overhead projector supplies, as it's the same thing as those plastic sheets that teachers would use in the 80s to photocopy lessons and then write on. These wouldn't be large enough to line a cake ring, but may suit your needs for chocolate work.
Otherwise, to buy it in person you need to find a specialty gourmet baking/pastry store - the kind of place that has all sorts of molds and pans and weird ingredients for pastry work. (Not a Michaels, Williams-Sonoma type, more like a restaurant supply or warehouse.)
Cynthia is a trusted source on Bread/Baking.
A drafting (engineering) supply store. Very cheap, and they usually love it when baking and pastry people find their way through the door.
Really! That's excellent info boulangere I had no idea.
amysarah is a trusted home cook.
We order it in rolls (for graphic presentation, not baking) - comes in both sheets and rolls: http://www.misterart.com...~
Sam Flax also has it; not sure how they compare on acetate, but Flax is more expensive in general: http://www.samflaxny.com...
Pearl Paint probably also carries it.
One tip - acetate comes in different weights, like paper, and is priced accordingly. For your purpose, you probably only need the thin stuff.
Multiple regular-sized cakes > a big one.
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