I have bought mirrored acrylic from a few different places and have had mixed results. The best I ever got was from eBay, the same company as Dave recommended I think.
I would say do not buy from a company called 'Plastic Sheets'. I saw that they had great prices so I bought a sheet in every colour from them which came to around £70. They arrived with the grey mirrored backing quite scratched, a few were really scratched, as if someone had gone sledging on it!
I had quite a battle with them to get a refund and had to open a Paypal dispute in the end to get my money back (thank God I paid through Paypal). After this I read reviews of the company and many others have had similar experiences with difficulties getting refunds and calls and emails to the company being ignored.
So Iearned that if prices seem to good to be true they probably be are and even if the company seems big and decent from their website always check independent reviews.
So although the eBay one was more expensive than other 3 places I have tried, the acrylic was the best quality and had the nicest cut.
Hope this helps