Can you order plastic pieces that backpack shoulder straps fit into?

I would recommend checking with wherever you bought the backpack, or with Patagonia directly. From their website:

We guarantee everything we make. If you are not satisfied with one of our products at the time you receive it, or if one of our products does not perform to your satisfaction, return it to the store you bought it from or to Patagonia for a repair, replacement or refund. Damage due to wear and tear will be repaired at a reasonable charge.

That makes it very likely they have the parts available. I know many of the parts are available at camping stores, and sometimes even hardware stores, too.