They are manufactured chews with a non-nutritive substrate full of small nylon fibers that massage the gums and clean the teeth as the dog chews. They make a lot of different products, but the durable chews are the ones I use because Labs are VERY powerful chewers with long jaws that develop a lot of leverage. Nylabone durable chews. You want to get them big enough so that the dog gnaws sideways and doesn't crunch straight down.
Deer or elk antlers are a good alternative, especially if you have a friend who hunts or has land that deer live on - they shed their antlers after the rut and you can just walk around and pick them up. We have a friend with 100 acres in a rural GA county, he saves the ones he finds when he's out hunting, and when we go down to shoot on his land we scrounge them up ourselves. Some people train their dogs to hunt for them!
I use the thick base of the antlers and saw off the pointy bits just with an ordinary hand hack saw. The dogs dance around the workbench waiting for me to hand them the goodies!
The antlers sound pretty icky, but we’ll look into Nylabone chews. Ash makes a real mess with the rawhide ones.