The two main choices tend to be fire brick or clay pavers. Clay pavers (but definitely NOT concrete pavers) are said to last OK, so I bought some second-hand through eBay. I'd much prefer to re-use existing materials, and clay pavers are way cheaper than fire bricks in this country.
Because the Hebel insulating layer was nice and flat, it was easy to lay the hearth pavers directly over the Hebel blocks without any sand in between.
In this oven design, there is a course of pavers sitting upright on their long edge (that is, upright but lengthwise) forming the bottom part of the dome. The dome sits on top of the hearth pavers, so the layout of the hearth pavers will roughly determine the size of the dome. This photo should help illustrate how I laid things out:
My hearth is basically 5 pavers long, by 7 wide and narrows down to 6 wide at the front tunnel. I cut a couple of the hearth pavers at angles so the corners wouldn't project out past the curves of the dome. The photo shows the layer of upright pavers as well; they have been mortared on using high-temperature mortar that can be purchased at places like BBQ shops - I only needed a 1kg tub.
In laying out the pavers and sizing the dome, remember that the dome will be covered in insulation when it's finished - you will have to allow for that thickness.