Most cast iron surface plates come with a 3 point bearing pads. I have 2 surface plates, a 10"x18" no name, and a 24"x36" Challenge. Both have 3 load bearing points built in. Here's some text from the Challenge Precision website concerning their cast iron surface plates:
"Three point surface plates provide a precise reference surface for tool making, layout, or checking the accuracy of other surfaces. The three point bearing pads and rigid ribbed construction permit support by less accurate surfaces without disturbing the surface
accuracy. Wooden cover supplied with each plate."
Here's a link to the Challenge Precision cast iron surface plates:
http://www.challengeprecision.com/products/plates/threepoint.htm
The 24"x36" plate came from our machine shop at work. It weighs 475 lbs. It has spent it's entire 50+ yearlife on an angle iron stand with a 3/4" plywood top. You would have to run into the stand with a fork lift or Mack truck to tip it over