I need to replace part numbers A103311 and A66D814 on my Glacier Bay shower assembly (816-635G). Online, all I can find is the entire assembly. I just need two parts. Where do I get them?

You probably won't. They are Chinese imports and small replacement parts are typically not available. Have you checked at Home Depot, it's their brand.

Might try this from the Home Depot site.

http://www.homedepot.com/catalog/pdfImages/27/2768befb-825d-481b-8768-3a841726bcb1.pdf

Another possibility:

https://www.manualshelf.com/manual/glacier-bay/f1aa0005bnv/warranty-english.html