This translucent polycarbonate panels, and the aluminum framing system that holds them, are products of Polygal. You can find a construction supply distributor to supply them in your area.

Can polygal be used to create a roof over existing wood rafters?

No reason why not. Except depending on the rafter spacing, the specific Polygal product used, and the local snow load -- this particular product which is fluted, spans the long way -- up-and-down, not between rafters -- so cross-ways purlins are used. You can see them, spaced 2', while the rafters are then spaced 4'.