Message your local plastics supplier. If money's no object they will usually be able to hook you up with what ever you need (excluding the all but best & exotic stuff like 0.25 cast acrylic etc..).
They also will have a vast offcuts selection that can be had for a reasonable price.
Unusual acrylic like thin coloured pieces will usually sit in their offcuts section for ages so they'll be keen to get rid of them ( I've had companies chuck in loads of offcuts of a large size for free with big acrylic orders).
Amazon & eBay usually only have 3mm & up or lexan claiming to be acrylic (which is junk).
Sorry I can't rectangular suppliers as it depends on your location.
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