Hi all!
Just a little assistance with some terms.....
The insulative fire brick (IFBs) are the light, generally white ones and Thomas is right that they are great for the heat riser but not in situations where they will be continually abraded by wood.
As far as refractory firebrick goes (generally yellow/buff in color), you can find low, medium, and high duty versions out there. There isn't much low duty firebrick produced these days. Medium duty will more than meet RMH requirements and is more economic than the high duty.
Alsey, Whitaker Greer, and Harbison Walker International (HWI) are the main fire brick makers / distributors in North America. It is most likely that a local brick supplier will have one of these three brands.
HWI has its own distribution centers and an impressive array of refractory products, including castables. There is also some fire brick coming in from India and China these days.
In our experience as heater masons.... it doesn't really make sense to stock up on firebrick. We prefer to let local masonry supply shops supply the firebrick (and red brick) for projects and for us to have more specialized materials on hand. For us, this has evolved to stocking a full line of mass heater hardware which we have in stock and ship:
www.firespeaking.com/hardware
Hope this helps!
