I got my flats from Amazon. Cheap, likely Chinese, plastics. The "sellers" keep disappearing, but identical products keep popping up by others. Buy in packs, 6, 8, 10, 12... Expect they will cost you $2-3 each, so a pack will run you $18-36 depending on size.
They bleach well, take hot water well, drain for air drying (reasonably) well [I had to finish cleaning the holes out on a couple], they stack, and they've held up a couple years. If a neighbor forgets to bring one back, I'm only out a couple bucks.
I store mine stacked in the fridge, 4 high on a shelf. They will fit a duck egg, so birds laying extra large aren't a problem.
and you can't see it, but I marked mine with a black sharpie to create black bars on two sides. When I stack them, it makes a "stair" of black marks - helps ensure that they stay in order, oldest to newest. Have to do two sides because one flat will lay 6 across 5 deep, the next flat gets turned 90 degrees so 5 across 6 deep. Repeat repeat, repeat.
Once stacked 8 tall in a cooler and drove them 6 hours across state, didn't lose a single egg. (plastic "ice" blocks too). Big cooler, weighed more than I could easily handle alone once loaded. Got rid of it.
Hope that helps a little?