Now I'm feeling guilty that I might have used up the last 75kg fireclay in england when I built my dome
Can I recommend a good book: "Contemporary Ceramic Formulas" - John Conrad (published 1980 - so not that contemporary any more...). I used this for guidance when I made my dome mix and my insulation mix. It has an entire chapter on making refractory materials using clays - clearly from the days when it was the norm to build your own kiln.
This book has a range of recipes for firebrick, insulation brick, castable refractories, crucibles, kiln shelves and posts and 'kiln repair paste'. The bad news is that the majority of the recipes use fireclay
but there are some that don't. The non-fireclay recipes use ball clay (R101), ball clay and sodium silicate (R106), kaolin & ball clay (R109, R121), mullite (R114), mullite & ball clay (R116, R118, R119), kaolin & talc & spodumene (R127) or ball & china clay (R130).
Ths might give you an idea what you could substitute for the fireclay?
Tom Jaine recommends using a portland:lime:sand mortar for the oven bricks (1:1:6), but then he also recommends using engineering brick rather than firebrick.
Carl