I would reccomend AliExpress, From experience I can tell you, the most difficult thing to find will be antennas.
The chips as @Fade0 suggested, salvaging chips from any of the T5577 products, MOST of the antennas you will find will be disc shaped.
However you may find a few of the EM40xx products with rectangular antennas.
Just attach the T5 to the EM antenna
Due to physics, you will find the 125kHz antennas will be either thin and large, or more Chode like (Short and fat)