I have no idea how you managed to not find it on Amazon.

https://preview.redd.it/d7mrhq0rrkpa1.jpeg?width=1080&format=pjpg&auto=webp&v=enabled&s=be6f698178ca077f1d64cedbd3ec8b3814ea03b7

Theres plenty https://www.amazon.com/MG-Chemicals-Copper-Board-Double/dp/B008OAFNWS/ref=mp_s_a_1_7?crid=8IMMWY8M5WTK&keywords=fr-4+copper+clad+1oz+500&qid=1679596790&sprefix=fr-4+copper+clad+1oz+500%2Caps%2C214&sr=8-7

I bought this one from Amazon, MG Chemicals copper clad 12"x 12"

MG Chemicals 555 Copper Clad Board

It cuts pretty easily, though it is fiberglass so you need to deal with the dust appropriately.

I stuck a magnetic sheet to the bottom and use it on a cast AL 6mm plate, but it seems rigid enough to help with a flat bed.

I'm in Canada, so there aren't as many options, but this works perfectly. I scuffed the entire surface with a green scotchbrite to help with adhesion, and then pre-heat 5 degrees hotter than i do with PEI (60c for FR4).

All of that being said, I never found a need for it with PLA or PETG, I got it for Nylon. And the weight really doesn't make a difference with the E5, so if glass is working for you, i would just stick with that personally. I love my textured and flat PEI for PLA, it works amazing and everything pops off when you flex the plate.