This questions has spawned a lot in several of my Facebook Groups lately and here are some answers:
We buy ours from https://www.zellwin.com/Egg_Flats_s/72.htm
But there is also Amazon and some other places https://www.amazon.com/Nasco-Super-Pocket-Egg-Flats/dp/B077XJN8V2
If you have a preferred Egg Crate vendor do feel free to share here!