There are a couple ways to attack the problem. If you can't get an exact replacement, get a larger piece and cut it down to fit. You will need to rent or borrow a tile saw. Or you could build a threshold out of several pieces of tile. You would adhere the tile to the concrete with thin set mortar. Before you do that, it would be good to understand why the thresholds are failing in the first place. Were they installed incorrectly? is the underlying concrete cracking? Is there any kind of HOA restriction that says you must use ceramic? Maybe a metal or wood threshold would perform better?