Where can I find 4 ft by 8 ft sheets of vertical siding?

Not sure I understand your question, are you looking for 4x8 sheets of vinyl or aluminum siding? Or some kind of wood siding like a T-111 siding? Don't think you can buy a 4x8 sheet of aluminum or vinyl siding, it doesn't work like that. Your local building center should have some materials for you depending on what you are needing.

Veritcle Aluminum Siding Panels Semi-Coregated Wood Grain White Finish

Vinyl or aluminum vertical siding comes in 12''18'' or 24'' widths.
Would have to get it at a roofing and siding supply house.

Have not seen 4' by 8'.

Chuck