The best place to buy new fire brick is at a masonry supply house.
Also the best spot to find fireclay.
Home depot or any box store would be a last choice for firebrick and fireclay, but the first place to buy plain clay bricks and sand for your mortar.
Used firebricks are definitely out there, on craigs list and other similar places.
Depending on your location, sawmills , pulp mills , refinery's all have kilns and they all have firebricks sitting around.
My first couple of hundred were free from a sawmill owner after I explained what I wanted to do with them!
Also: companies that install wood stoves and inserts usually have a junk pile out back which includes firebrick in various states. Chat them up and it's yours for the taking -- saves them the cost of trucking to landfill, which is by the pound/kilo for businesses and it ain't cheap.