Wire Size for 100A Subpanel

This is a continuation of my basement wiring topic, this time about the service wire feeding the 100A subpanel.

The previous owner chose to install the subpanel to address the lack of breaker space in the main panel. The subpanel is 4" away from the main panel. I have determined that to effectively use the subpanel, I need to add a ground bar because both existing bars are on left/right side of the box and both will need to be used for the neutral pigtails on all the AFCI breakers (required). The mounting holes for the ground bar kit (GE Powermark Gold Load Center) are, you guessed it, on the opposite side of the panel from where the service feed enters the subpanel and the ground is terminated. So the ground will not be long enough to reach the other side of the panel and I will need to run a whole new feed cable from the main panel to the subpanel.

For new cable, based on what I've read, the chart shows that I need to run 1-1-1-3 copper or 1/0-1/0-1/0-? aluminum to guarantee 100A at 60deg. Am I interpreting this correctly?

What size would the ground need to be in that 1/0 aluminum cable?

These cable sizes don't seem to be big box stocked (by the foot), are these something I can readily find at electric supply houses or online? Any suggested links? I would only need like 4 or 5 feet.