I've found some information on where the outer viaduct curves can be obtained.
One set is the Plarail Advance L0 Maglev Set
Maglev Set
It also came in the Plarail Thank you For Dream Express Shinkansen Hikari Set
Hikari Set
Unfortunately for us. These sets typically tend to be very expensive, especially the Hikari one, and according to this source, it is the only two sets that contain the outer viaduct curves.
Hopefully this helps anyone looking for these pieces
One set is the Plarail Advance L0 Maglev Set
Maglev Set
It also came in the Plarail Thank you For Dream Express Shinkansen Hikari Set
Hikari Set
Unfortunately for us. These sets typically tend to be very expensive, especially the Hikari one, and according to this source, it is the only two sets that contain the outer viaduct curves.
Hopefully this helps anyone looking for these pieces