I second that. I got mine on amazon before I got my phone and it's exactly what I was looking for. It was around $15, thin, decent looking...

Edit: https://www.amazon.com/gp/product/B0789S2LZH/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1

I have this Spigen Neo Hybrid case (rubber inside, plastic shell): https://smile.amazon.com/gp/product/B0789S2LZH/

As for glass SP, I would look on AliExpress for the UV glue glass case friendly protectors. You can get for relatively cheap. XDA has some threads about it: https://forum.xda-developers.com/galaxy-s9-plus/accessories/aliexpress-liquid-dispertion-tech-t3828245/post78708449#post78708449