You can often find it in Home Depot, sold as "stair tread". Look where they have the other stair stuff.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)
Next: 2022 Fully-custom buildout (Ford Transit EcoBoost AWD)