Building a pizza oven dome depends on a number of factors; the materials you are using to build the oven, the type of pizza oven you are building, and the materials available.
If you are building an igloo oven, then the dome can be done with sand or wooden support, or any other dome shaped materials that can support the dome effectively. Sand is the cost effective as it brings out the shape effectively and it with less cost.
If you are doing a barrel oven, then the dome can be some with a formwork made of wood, metal, or you can use sand as well as any locally sourced materials that have the shape of the barrel.
With the supporting material in place, you will simply complete the dome with the material you are using which might be fire bricks, cobs, clay, or locally collected natural stones.
Further information: How to construct a pizza oven dome out of brick, How to construct a pizza oven dome out of cob, and How to build a pizza oven dome