Where can I find the stair treads on line?
Not sure what you want. However, you can buy anti-slip gripper strips that fasten to stairs at Home Depot. Assuming these are outside stairs.
the Home depot has them. You can get them to ship as well if you don't have a local store
I went on line to find out who sells slip-free stair treads. Home Depot sells them and Menard's does too. You can go to their on line stores and they will ship them to you. Amazon also has them.
Here are 5 products reviewed: https://www.top5reviewed.com/stair-treads/