I always just go to my local art store. True, not 
all stores carry larger pieces, but some do carry 
large single sheets. The size I have found near 
me is 19x23...and they may come larger than that. 
Some places will also sell rolls of acetate which 
can be 20 or 30 feet long. And if a store carries 
acetate in general but not the size you need, 
chances are they can order it for you.