Hi Catherine:
I see you are in Mo. I don't know what part of the state you are in and what the nearest large town that has a masonry supply house.
A quick look on line showed me a place on the west side of the state here is a link
http://leessummitblocknstone.com/
Bunches more on the east side here is just one in St Louis
https://themasonrystore.com/
Masonry supply is where you would find Firebrick, full & split's, if your lucky they have insulated firebricks as well ,sacks of fireclay, builders sand, plain clay bricks .