I'll be after quite a large surface plate myself down the track, around 3000mm x 600mm x 25-50mm, and have looked at the various options such as float glass, granite slabs, etc, and the costs of these materials when sold specifically as surface plates is astronomical. But I have heard off several people that ordinary granite slabs for masonry or for kitchen benchtops are more than adequate as they have been ground flat to a fairly precise finished dimension. So when the time comes, I might look for something along this avenue, even a damaged top that just needs to be cut at one end.
cheers, Ian