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.

