Well, I have no idea what happened to my previous answer.... sorry !
All my original fireplaces were removed by previous owners... it was all the rage back in the 1970's.... doh !
I think you'd need to go to some reclamation yards or salvage places to get hold of the original tiles. If the ones on your hearth are cracked, then they won't be much use in a fireplace and you'll need to replace them... there are some tile manufacturers now making *repro* looking stuff though. 6 x 2 is an unusual size - are you talking about 6" by 2" ?