You might want to add where you are located, shipping for a marble slab could negate a good price!
If you are not too fussy on how the marble slab is finished on the edges or size, shape or colour, just go to a local stone cutters and see if they will let you go through their off cuts. They will have anything up to a meter by a meter in their bin. I just picked up some pieces for our lab to go under the balances to dampen vibrations from other equipment. There was a sweet piece in the skip that was perfectly square 1 x 1 meters, the guy let us take what we wanted for free. They would probably edge it for you if you paid them a little.
Tom's idea is right on. A countertop fabricator friend of mine made me a granite slab and he even finished handholds for me so it's easy to pick up and move.
I actually picked up a free piece of countertop from the trash bin at a countertop place, the only problem is that it's not as big as I would like, and it's pretty thick (and heavy).