Granite Slabs

https://www.amazon.co.uk/HHIP-4401-1597-Granite-Surface-Plate/dp/B08BXMX46W/ref=sr_1_2?asc_source=01H8HFYCRM99TJ9FED7FSB1ZXC&crid=18KGO8XAFZZ7F&dib=eyJ2IjoiMSJ9.232CKKt0hr-jungdZugkjZ7qNo185dO8bd0NCmJGuEcGZHhLVVtqGBZU1r-B34P8gn0Gjbv_CBNTREyJ5hzmASRzXidIC308gb8LCXmRle2C5SCbcMrap21h9thdmFTF8SUVgQtPZF0LSET-yCL6PJnBkedAq4M4dLnJ3KKNHY-qej5FAUjkjBHDgDzpCuXcBkOkjUJN_3hrAIz7oEhwfzHjcykLIr9he9yZJ9q8_o14oarB567arAL2P8rWrYZQbnbJfh-pVAqNTNW6JLZEMRIb-WS5v1jnmQWVPcmyTq0.VxXdDrpTsovlgD5A-YhqYzXRSNm62KG5W9IE_dCj4NU&dib_tag=se&keywords=engineers%2Bsurface%2Bplate&qid=1764097976&sprefix=Engineers%2BPlate%2Caps%2C387&sr=8-2&tag=snxgb19-21&ufe=app_do%3Aamzn1.fos.95fd378e-6299-4723-b1f1-3952ffba15af&th=1

https://www.ebay.co.uk/itm/365966181421?_skw=Granite+Surface+Plate&itmmeta=01KAY7990GBRKVA4NR48D31503&hash=item553548c42d:g:-qQAAeSwqUVonOL6&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1eJQZCaXj8UaQUEpHeLXThzTsDOeyKnOfwR5RgLI19LQdiAeKX4rclm1IjzfHFROUpqE83xDEjA4wzAzQnHaHIZyDenV2dY1iQO%2B9M7810k1%2Fr2EZ29rph1qArLOWMloV33FUW3jNmae73FIrkd6CNBXHbkIh8i6hgEQJng2rwjkHnndhXvY3osz7uhOtrRoITWoyuygyVNBNpCmknQNYk%2B5pOJAvn7zCKfR2H3R6Z3VBgL9qPQHRPBVc8dtihvW4y%2BZNDWsUqcUOqK4lV4DcmgmSWep4UknPVgdD6Rn9pX4g%3D%3D%7Ctkp%3ABk9SR8SQpcfXZg

Thicker is better, common sense of course should be applied. 
if you will be ordering from granite cutting shop, don’t tell them that you’ll use it for sound system. Tell them you’ll use them as kitchen cutting boards so it won’t cost you a fortune. This strategy actually helped me to save a hundred. 

When we moved house around 10 years ago, I had solid granite kitchen counters custom cut. I had to buy either a sheet of granite (which is quite large) or a half sheet, so I bought a full sheet and had them cut and polish two smaller slabs to my specification to use as bases under two of my turntables.  It worked out very well.

Suggest that you contact a granite business - they will always have part sheets left over from jobs and should be able to supply cut down pieces to your specification.  Be aware that they will be fairly heavy and need a good solid base underneath them - it can even be as simple as a couple of standard 8x8x16 concrete blocks turned on end or it can be something more aesthetically pleasing.

I'd try to get scraps/odd cut pieces at a discount. 

I agree with goofyfoot.  Most granite shops will have leftover pieces from builds.

I built an isolation platform for my turntable.  The process was go to the granite shop, let them know what you are doing, they will show you what they have available.  The shop where I purchased mine charged about 10 for the piece if you want it cut they will do that most likely my guy charged 8 per cut,  granite comes in either 3cm or 2cm slabs.  They may charge a bit extra if you want the edges smoothed off.

after that I sandwiched closed cell foam between the granite and and a plywood base.  I installed spikes in the plywood.  Or sorbothane feet whatever suits you.