Yes you can, if you go on line and type in 'metal bases for greenhouses' several come up.

I have just built one that size on my allotment and used breeze blocks and fencing posts instead of a base as this is much cheaper.

I think you will have to know the make of the greenhouse to buy a metal base, because the size 8x12 is only a nominal size, and can vary in different manufacturers, so once you know the make you can get the exact size you need, Derek.