You could likely use pallets. You'd be able to cut and build those easily. They're usually light weight and not so thick for cutting through. One pallet would provide numerous possibilities as you'd has a lot of wood available for framing and stabilizing for stability....Just a suggestion.
A stair from what to what?
I found a few under the explore projects tab at the top of the window. Here is the address for building all kinds of pet things, including stairs and ramps.
Have fun, I hope you find one you want to build!
These are my favorite: