You can find all the components you need at Escapa.
Like the Xlc All Ride SP-O01 Seatpost, available in different lengths and diameters.
Material: 2014-T6 aluminum, "one-piece construction."
Cold-forged.
Offset: 0 mm.
Patent head with 2 bolts, 'micro-adjustment'.
Length: 300/350/400 mm.
Diameter: Ø27.2/30.9/31.6 mm.
Weight: approx. 250 g (Ø31.6 x 350 mm).