Looking for fire brick sources

Hey All,
So I've caught the rocket mass heater bug and I' starting to gather up materials.  I've decided that I would like to use fire brick for my riser tube.  Locally all I can find are fire bricks that measure 4 1/2 X 9 X 1 /14.  I don't think that's what I want.    

What is the best size brick for a riser tube and where might I source some?

Thanks
Rich

What you have found are called splits .. 1/2 thickness of regular fire bricks.  They are exactly what are recommended for building a riser.  There are better things such as insulated fire brick or carbon fiber boards , vacuum tubes, but splits with  rock wool wrapped around it are a good choice.

Masonry supply houses generally stock full and split fire brick, some even carry the highly desirable insulated fire brick, they also may have fire clay available.  Don't know your  location but an example is White Block in Spokane Valley  WA.  stocks fire brick and fire clay at very good prices.