Probably these things that Tractor Supply Company sells (or similar ones from other sources.)
https://www.tractorsupply.com/tsc/product/tractor-supply-pine-pellet-stall-bedding-40-lb
They look like a bag of pellets (almost like short pieces of pencils or dowels), but they are made of pine sawdust pressed together. When they get wet, even a little bit, the pellets fall apart and look like sawdust again.
Pellets like that are sometimes sold for kitty litter, or for bedding in horse stalls, or for various other purposes. The ones for horse stalls are usually much cheaper than the ones for kitty litter, even if they are pretty much the same thing. Any ones that are sold for animal bedding should be fine for chickens.
Pellets that look pretty much the same are also sold for burning in woodstoves or smokers. They might also work, but check the label carefully-- they might have things added that are better for burning but worse for chicken bedding.