Look for selvedge denim for jeans and duck canvas for other pants. I’ve had some luck with Naked and Famous, but they can be a bit pricy.

There’s quite a range of quality and durability in denim, but it can be extremely durable. It was associated with physical labor in the US for a long time and especially in its old West (cowboys, railway workers, etc). You can still find a pair of jeans that will last you the rest of your life and then be sold in a vintage store, but they’re not cheap. Cargo pants have a similar range but I think they’re typically made of duck canvas.

The elastic in underwear lasts a lot longer if you don’t put them in the dryer. Unless you regularly enjoy the sound of a buckle hitting the drum over and over, it’ll last a while in a belt.