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Before you start searching online retailers, I'd look locally. Grab a phonebook or online equivalent and search for plastics shops.
Shipping on a sheet that large will be incredible.
home depot carries acrylic but alot of them dont recognize it by the name acrylic go to home depot and ask them where there plexi glass is theyll point you in the right direction.
in thst case your best bet is to look up a glass repair shop I've never had to go to far because my dad owns a glass company so he can order custom sizes of glass to your size and thickness they can also do that for you.