They can be installed horizontally on the sides. My corrugated poly house is Quonset style, so there is no telling where roof and side begin and end. that part is vertical of course, so the water runs down the sides, but the polycarb end walls and doors are horizontal with no issues. (it's also that way by design--the panels are attached to aluminum framing).

You can buy corrugated polycarb at the two large home improvement store chains. The orange one carries a brand which also supplies a ridge cap which will seat itself on their polycarb product. The blue store's polycarb supplier does not have ridge cap material and the ridge cap from the other brand does not fit. 8 ft x 26 in panels are around $20. You an also buy 12 ft panels.