Where do you find fire bricks?
Some retailers sell them as such but they tend to be expensive, upwards of almost $10 apiece. My understanding is that they aren't necessarily special from regular, solid bricks. If it's not obvious, don't bother with the kind that have holes in them. I'd avoid heavily pitted or textured designs. Those might attract pockets of heat and crack.
I bought some at a place that sells reclaimed construction material. They had piles of old brick laying around, taken from various buildings. I just picked a few that were solid rectangles. Less than $1 each.
I played with them for a couple years but gave up on them due to impatience. They do block heat for a nice 2-zone setup, but at the expense of being a
large heatsink. More fuel, more time and so on.
I may come back to them, but only after doing something like combining 2 char baskets into one large basket, for more heat.
ProTip: Wrap them in foil first, to keep the inevitable grease off of them.