Where can you buy a replacement ic chip cable that is on the digitizer

I presume you mean the sensor for the Smart Cover...

Normally you de-solder it from the original digitizer and transfer it to the replacement. I don't know if it is sold separately but do a Google search for "ipad mini 4 smart cover sensor"

According to iFixit:

Please Note: You will need to swap the sensor flex cable assembly from your original front panel. This procedure requires micro soldering and board level repair skills. If you don’t know what this means you may not have the knowledge or tools to install this part. If you’re curious and want to know more about getting into micro soldering, here’s some good reading and some good watching to get you started.