In several of the welding books I have it says you need to insulate
your work from the steel plate used for support or the plate will rob
the working area from heat needed to create a good weld. I have a
triton sawhorse that I plan to hold the support plate with, but thought
I needed a few fire bricks on top of that to insulate the work.

As far as the fire brick - I have spoke to a number of fireplace,
masonry and HD type stores. It's all pre-fab large sheets now, no
indivual bricks. Nothing like progress..

Still looking though...