I know firebrick is used, but could other things be used like a large piece of slate? Very new to all this so any help appreciated.
As suggested by PMcCool, soapstone would be a great choice for a hearth. It's one of the best materials for absorbing and radiating heat, which is why it's used in the best wood stoves and fireplaces. It has the added advantage of being almost completely non-absorptive, so you don't have to worry about spills as much -- not that you really have to worry about this a lot anyway. It's also very DIY friendly and can be tooled with ordinary carbide woodworking tools. That said, it ain't cheap and isn't very widely available. (Nobody stocks it in my neck of the woods.)
But I suggest staying away from slab products, soapstone or otherwise, for hearths because if a slab hearth develops a problem, you gotta tear the oven down to replace it. With bricks or tiles, you can easily replace them individually without dismantling the oven. Soapstone bricks and tiles are available from several sources.Look here: http://www.soapstones.com/store/buy_soapstone_firebricks.htmland here:http://www.vermontsoapstone.com/soapstone_tile.asp Note that even the tile is $15 / sq ft. and a lot more expensive than firebrick.
If you still wanna use a slab, I'd suggest looking into granite. You may be able to get a deal on leftovers from a local countertop manufacturer. Even so, I'd cut it onto smaller segments to facilitate later repairs.
But firebrick works well, and is pretty darn cheap compared to the other options. Just curious -- why are you shying away from firebrick?
asking if other things are used for the cooking surface. I was thinking one solid cooking surface, but like you said if something happened to the cooking surface it'd be easier to fix a brick that a slab. Asking because I don't know and this seems to be one of the places to find out. Thanks for the soapstone links.