Where can I find parts for attic pulldown stairs?
I suspect you'll need to go directly to the manufacturer for this one. The only thing I've seen from the HI stores are fold down style of stairs.

For your pull down style, the first result I found in a Google search (so no affiliation or endorsement) is Bessler Stairway Company. They have parts for door and stringer spring tension drums, pulleys for the door cables, and the cables for the door and stringers. If you cannot identify your model, you may want to contact a manufacturer to see if their parts are generic or interchangeable with other manufacturers, or if they can help identify your model.