I have a Regency Wood burning fireplace insert. Some of the fire bricks are cracked / crumbling. Looks like they are mostly standard size 1 1/4 x 4.5 x 9', but one is 1 1/4 x 4 x 9, and one is 1 1/4 x 1 3/4 x 9. Does somebody sell non standard sizes or can they be cut?

Are all standard size firebricks of similar quality, or are there ratings or something I need to look for?