The inside of the oven needs to use bricks that will heat up, stay hot and not break, explode or spall over time.
Lots of people use fire bricks (aka refractory bricks, fireplace bricks, or heat resistant bricks). Vitcas and www.kilnlinings.co.uk are good sources of these in the UK. They start at around £1.30 each and you’ll need about 200 of them, so this is going to be one of the most expensive parts of the build. Firebricks are made from friecaly and have a high Aluminium content to keep the heat.
Old solid clay bricks will work just as well and can be bought from architectural scrapyards (ironically our local yard was selling them for £1.50each – more than the firebricks!) or scrounged for free. We got ours from a bungalow being demolished round the corner, finding solids with no frogs! the frog is the indentation in a brick.
Old Storage heater bricks can also be used – they are really heavy and store heat well – annoyingly I spotted a load after we had finished our build!