I'm probably doing this the hard way, but is there an easy way to create a shape that lines up exactly with the path on the left and right side?
I used the pen tool to trace out the left side of the shirt and collar, and did the same with the right side, but I'm wondering if there's an easier way to create the middle part (the part with buttons) than using the pen tool to trace along the path.
You may simply create any shape that covers all that part you want to have shape of (may be just a rectangle) and you subtract the shapes you have already created from the rectangle by using Pathfinder (Window -> Pathfinder). There is a function Minus Front that will deal with it. After subtracting both left and right sides of the shirt you will get the middle part. You can get more about shape cutting it in here: https://y-designs.com/tutorials/illustrator-tutorial-cut-out-a-shape-with-another-shape/
If I am understanding your question correctly, you could use Illustrator’s Blend tool.
Note that you may have to use the Scissors tool to temporarily disconnect your two paths before blending.
Select the two paths, go to Object > Blend > Make (read the docs to learn how to use the actual Blend toolbar tool).
With the blend selected, go to Object > Expand...; leave the default options and click OK.
With the expanded object selected, ungroup the paths (Object > Ungroup) so you can easily work with your newly-created middle path.
That’s one way of doing it.
With the non-expanded blend selected, go to Object > Blend > Blend Options... and you can play with the options to control how many blend steps get added:
