"The Children's Place" clothing has adjustable waistbands
Gap Kids also has adjustable waist pants. Old navy(the same company) also has this in certain pant styles.
Arizona brand available at JCPenney also has adjustable waists. I thinkk the ones at "Justice" or "Limited Too" also has them.
My little boy just got out of these. Arizona Jean Co. (sold at JCPenney) has adjustable elastic (the kind with the buttons) up to 5T.
Circo at Target has adjustable waist. So does Gymboree!
I think almost all do these days. I've bought some from Old Navy, Target, Gymboree, JCPenney, Children's Place,...