I reuse the plastic bowls from microwave meals. They're good for pots up to 5.5-inch / 14 cm in diameter. The trays for take-out food are handy, too. I have a couple trays from large orders of Culver's cheese curds that are currently being used to growing sphagnum moss. I don't expect them to last more than 1 or 2 seasons at the most, but they're good while they last.
: If you don't need deep trays, these
white 1020 trays work well. They're more expensive than the standard trays, but they're good for flytraps in smaller pots.
Edit -- After a bit of Googling, I found these:
Mondi Black & White Propagation Tray 1020 No Holes
https://www.discount-hydro.net/mondi-bl ... -no-holes/
https://mondiproducts.com/product/clean ... -no-holes/
They aren't completely white, but the black exterior won't soak up anywhere near
as much heat as all-black trays.